ClientSession can be woken up in order to destruct faster (closes #20)

The way that the ClientSession background thread worked before was that it
unconditionally slept for m_check_interval to throttle the check events. This
works fine during normal execution, but it's a big problem for destruction: the
destructor correctly joins the thread and waits for it to finish, but that means
that the destructor can take up to m_check_interval seconds to execute. This is
not ideal: the destructor runs when the program shuts down, which can mean that
the ClientSession can essentially "lock up" shutdown for a long time without
doing any work.

The fix is to not use unconditional sleep, but to use a std::condition_variable
instead: the condition variable can sleep for a given number of time just like
std::this_thread::sleep_for, but it can also be woken up if needed. This change
wakes the thread up when calling ClientSession::stop(), and the condition
variable reads m_is_running to figure out if it should wake up and exit. This
change makes it so that ClientSession::stop() (and therefore the destructor)
finishes (essentially) instantly, regardless of the value for m_check_interval.

src/client_session.cpp

@@ -74,6 +74,7 @@ void ClientSession::stop() {
   unique_lock<mutex> locker(this->m_run_check);
   if (this->m_is_running == true) {
     this->m_is_running = false;
+    m_monitor.notify_one();
     locker.unlock();
     this->m_daemon_thread.join();
   } else {
@@ -107,7 +108,13 @@ bool ClientSession::get_is_background() {
 
 void ClientSession::run() {
   do {
-    this_thread::sleep_for(chrono::milliseconds(this->m_check_interval));
+    {
+      unique_lock<mutex> guard(this->m_run_check);
+
+      m_monitor.wait_for(guard, chrono::milliseconds(this->m_check_interval), [&] {
+        return !this->m_is_running;
+      });
+    }
 
     unsigned long long check_time = Utils::get_unix_epoch_ms();
     unsigned long long range = this->m_is_background ? this->m_background_timeout : this->m_foreground_timeout;

src/client_session.hpp

@@ -58,6 +58,7 @@ class ClientSession {
 
   // Daemon
   thread m_daemon_thread;
+  condition_variable m_monitor;
   mutex m_run_check;
   mutex m_safe_get;
   bool m_is_running;