heartbeat thread should not call transport.disconnect_socket()

[mikebonnet]

Python sockets are not thread-safe. The receiver_thread created in BaseTransport.start() should be the only thing interacting directly with the socket, unless explicit locks are involved. When the HeartbeatListener detects a heartbeat timeout, it calls transport.disconnect_socket() from the heartbeat thread. This closes and deallocates the socket while the receiver_thread is still attempting to read from it. This can result in memory corruption and segfaults.