Protect m_timer under x_runTimer

For avoiding this warning from ThreadSanitizer:
==================
WARNING: ThreadSanitizer: data race (pid=72207)
  Write of size 8 at 0x7ffca08dd960 by thread T30:
    #0 std::enable_if<std::__and_<std::__not_<std::__is_tuple_like<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >*> >, std::is_move_constructible<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >*>, std::is_move_assignable<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >*> >::value, void>::type std::swap<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >*>(boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >*&, boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >*&) /usr/include/c++/7/bits/move.h:199 (eth+0x00000057b9f1)
    #1 std::unique_ptr<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >, std::default_delete<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > > > >::reset(boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >*) /usr/include/c++/7/bits/unique_ptr.h:374 (eth+0x00000057b9f1)
    #2 dev::p2p::Host::run(boost::system::error_code const&) /home/yh/src/cpp-ethereum-vptr/libp2p/Host.cpp:665 (eth+0x00000057b9f1)
    #3 operator() /home/yh/src/cpp-ethereum-vptr/libp2p/Host.cpp:720 (eth+0x00000057c426)
    #4 operator() /home/yh/.hunter/_Base/b96750b/a5f7296/9d3cb74/Install/include/boost/asio/detail/bind_handler.hpp:47 (eth+0x00000057c426)
    #5 asio_handler_invoke<boost::asio::detail::binder1<dev::p2p::Host::run(const boost::system::error_code&)::<lambda(const boost::system::error_code&)>, boost::system::error_code> > /home/yh/.hunter/_Base/b96750b/a5f7296/9d3cb74/Install/include/boost/asio/handler_invoke_hook.hpp:69 (eth+0x00000057c426)
    #6 invoke<boost::asio::detail::binder1<dev::p2p::Host::run(const boost::system::error_code&)::<lambda(const boost::system::error_code&)>, boost::system::error_code>, dev::p2p::Host::run(const boost::system::error_code&)::<lambda(const boost::system::error_code&)> > /home/yh/.hunter/_Base/b96750b/a5f7296/9d3cb74/Install/include/boost/asio/detail/handler_invoke_helpers.hpp:37 (eth+0x00000057c426) #7 do_complete /home/yh/.hunter/_Base/b96750b/a5f7296/9d3cb74/Install/include/boost/asio/detail/wait_handler.hpp:70
(eth+0x00000057c426)
    #8 boost::asio::detail::task_io_service_operation::complete(boost::asio::detail::task_io_service&, boost::system::error_code const&, unsigned long) /home/yh/.hunter/_Base/b96750b/a5f7296/9d3cb74/Install/include/boost/asio/detail/task_io_service_operation.hpp:38 (eth+0x00000058c145)
    #9 boost::asio::detail::task_io_service::do_run_one(boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex>&, boost::asio::detail::task_io_service_thread_info&, boost::system::error_code const&) /home/yh/.hunter/_Base/b96750b/a5f7296/9d3cb74/Install/include/boost/asio/detail/impl/task_io_service.ipp:372 (eth+0x00000058c145)
    #10 boost::asio::detail::task_io_service::run(boost::system::error_code&) /home/yh/.hunter/_Base/b96750b/a5f7296/9d3cb74/Install/include/boost/asio/detail/impl/task_io_service.ipp:149 (eth+0x00000058c145)
    #11 boost::asio::io_service::run() /home/yh/.hunter/_Base/b96750b/a5f7296/9d3cb74/Install/include/boost/asio/impl/io_service.ipp:59 (eth+0x00000057168e)
    #12 dev::p2p::Host::doWork() /home/yh/src/cpp-ethereum-vptr/libp2p/Host.cpp:774 (eth+0x00000057168e)
    #13 dev::Worker::workLoop() /home/yh/src/cpp-ethereum-vptr/libdevcore/Worker.cpp:140 (eth+0x0000004a9f64)

  Previous read of size 8 at 0x7ffca08dd960 by main thread:
    #0 std::__uniq_ptr_impl<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >, std::default_delete<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > > > >::_M_ptr() const /usr/include/c++/7/bits/unique_ptr.h:147 (eth+0x0000005701cd)
    #1 std::unique_ptr<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >, std::default_delete<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > > > >::get() const /usr/include/c++/7/bits/unique_ptr.h:337 (eth+0x0000005701cd)
    #2 std::unique_ptr<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > >, std::default_delete<boost::asio::basic_deadline_timer<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime>, boost::asio::deadline_timer_service<boost::posix_time::ptime, boost::asio::time_traits<boost::posix_time::ptime> > > > >::operator bool() const /usr/include/c++/7/bits/unique_ptr.h:351 (eth+0x0000005701cd)
    #3 dev::p2p::Host::stop() /home/yh/src/cpp-ethereum-vptr/libp2p/Host.cpp:152 (eth+0x0000005701cd)
    #4 dev::WebThreeDirect::~WebThreeDirect() /home/yh/src/cpp-ethereum-vptr/libwebthree/WebThree.cpp:90 (eth+0x000000456f75)
    #5 main /home/yh/src/cpp-ethereum-vptr/eth/main.cpp:937 (eth+0x0000000c0ff2)

  Location is stack of main thread.

libp2p/Host.cpp

@@ -149,8 +149,15 @@ void Host::stop()
 		return;
 
 	// wait for m_timer to reset (indicating network scheduler has stopped)
-	while (!!m_timer)
-		this_thread::sleep_for(chrono::milliseconds(50));
+	{
+		std::unique_lock<std::mutex> l{x_runTimer};
+		while (!!m_timer)
+		{
+			l.unlock();
+			this_thread::sleep_for(chrono::milliseconds(50));
+			l.lock();
+		}
+	}
 
 	// stop worker thread
 	if (isWorking())
@@ -659,7 +666,8 @@ void Host::run(boost::system::error_code const&)
 		m_ioService.stop();
 
 		// resetting timer signals network that nothing else can be scheduled to run
-		m_timer.reset();
+		DEV_GUARDED(x_runTimer)
+			m_timer.reset();
 		return;
 	}