Fix frequent segmentation fault of Python tests by refactoring ExecutorService

[BewareMyPower]

Fixes #11635

Motivation

Python tests failed very often with segmentation fault. It can be reproduced easily based on the apachepulsar/pulsar-build:ubuntu-16.04-pb3 image in local env. After dumping the core file and running gdb python core to debug, we can see the following backtrace.

#0  0x00007f655e5e8d0d in std::__atomic_base<long>::load (__m=std::memory_order_seq_cst, this=0xb8) at /usr/include/c++/5/bits/atomic_base.h:396
#1  std::__atomic_base<long>::operator long (this=0xb8) at /usr/include/c++/5/bits/atomic_base.h:259
#2  0x00007f655e5e6348 in boost::asio::detail::task_io_service::run (this=0x0, ec=...) at /usr/include/boost/asio/detail/impl/task_io_service.ipp:136
#3  0x00007f655e5e6cc4 in boost::asio::io_service::run (this=0x1f11990) at /usr/include/boost/asio/impl/io_service.ipp:59
#4  0x00007f655e5e32d0 in pulsar::ExecutorService::startWorker (this=0x1f11130, io_service=std::shared_ptr (count 2, weak 0) 0x1e8ba00) at /pulsar/pulsar-client-cpp/lib/ExecutorService.cc:37

The io_service object is not null, but the internal impl_ field, whose type is a task_io_service pointer, is null (see (this=0x0 of #2).

The cause is io_service::run is called in a dependent thread, while the destructor of ExecutorService is called in another thread. After the ExecutorService is destructed, the internal thread field worker_ and the io_service field will both be invalid to access. io_service::run should not be called after that.

Modifications

Refactor the ExecutorService. Since it's not copyable, it's redundant to store io_service and io_service::work as smart pointers. In addition, the thread that runs io_service is never used, just detach it when ExecutorService is created.

The key point is to check whether ExecutorService#close is called in the thread by examining the atomic boolean value closed_. Besides, the shared pointer is captured in the thread so that the ExecutorService's lifetime could be extended.

[BewareMyPower]

Though it has been verified in my local env for many times, I'm still looking forward to what will happen in CI. Please don't rerun the cpp tests if it failed.

[lhotari]

Great job @BewareMyPower !

[BewareMyPower]

PeriodicTaskTest failed before running Python tests. And there're some compilation errors on CentOS 7. I'll fix them soon.