Use heartbeat_terminate_event as heartbeat loop exit condition to avoid a race condition

[Giz]

What can happen (and really does happen) without this small change is that:

• HeartbeatListener.on_disconnected is called;
• self.running is set to False;
• there is a switch to the heartbeat thread;
• self.running is evaluated and the while loop of HeartbeatListener.__heartbeat_loop exits;
• self.heartbeat_terminate_event.clear() is called;
• there is a switch to the previous thread;
• self.heartbeat_terminate_event.set() is called.

In this way on the next connection heartbeat_terminate_event will already be set and at the first opportunity the break on line 281 will be called interrupting the sending of heartbeats. When the server forces the disconnection, on_disconnected will be called setting heartbeat_terminate_event again. This recreates the previous situation where the sending of heatbeats is immediately stopped.

[Giz]

I added a second commit to move the heartbeat_terminate_event cleanup before the heartbeat thread starts. This is because there are cases in which on_disconnected is called without first calling on_connected, creating a situation similar to that described in the previous comment (the loop starts with heartbeat_terminate_event set, the heartbeat stops immediately, after a while the server closes the connection, on_disconnected is called, at the next connection heartbeat_terminate_event is already set).