Make client terminate gracefully

[JonnyKong]

Currently, when the client is terminated on SIGINT, Tracking::~Tracking() shows unsafe behavior:

edgeslam/src/Tracking.cc

Lines 2323 to 2330 in d548979

	Tracking::~Tracking(){
	keyframe_socket->~TcpSocket();
	frame_socket->~TcpSocket();
	map_socket->~TcpSocket();
	// This is just a dummy enqueue to unblock the wait_dequeue function in tcp_send()
	keyframe_queue.enqueue("exit");
	frame_queue.enqueue("exit");
	}

The sockets are destroyed, then "exit" messages are sent through the queue to signal keyframe_thread and frame_thread to terminate.

After the two threads are un-blocked in tcp_send(), they will then call TcpSocket::checkAlive() before terminating. This is unsafe since the sockets might have already been destroyed (it causes a segfault on my machine):

edgeslam/src/Tracking.cc

Line 2257 in d548979

if(!socketObject->checkAlive())

This pull request fixes this issue.

[dasda-asd]

Can the code run monocular mode, please?

[dasda-asd]

@JonnyKong

[dasda-asd]

@dkkarthik