Join the recovery threads.

[mojoX911]

Problem

In the Maker server, we sometimes launch the recovery thread. But we don't join them after completion. They should be joined.

Ex:

coinswap/src/maker/api.rs

Lines 478 to 483 in bab9614

	std::thread::spawn(move || {
	recover_from_swap(maker_clone, outgoings, incomings).unwrap();
	});
	// Clear the state value here
	*connection_state = ConnectionState::default();
	break;

Approach.

• Make a globally accessible thread pool in the Maker struct definition.
• Add the recovery thread as well as all other thread in this thread pool.
• Join all the threads from this pool at shutdown.