Fix race condition on exit of trpc threads

Fix race condition on exit of trpc threads

The messaging between the main thread and the trpc (outgoing connection)
threads allowed the trpc data to be cleaned up before the message queue
was empty, causing incorrect mutex behavior and seg faults.

This is (I hope!) solved adding an additional shutdown phase in which
the main thread indicates that it has recognized that the trpc thread
is done and that the trpc thread can safely exit.

So far, I have not seen a failure of the system to handle a peer
disconnecting. Prior to these changes, it failed every time with my
current setup.