prserv/serv: Improve exit handling

prserv/serv: Improve exit handling

Currently, I'm not sure how the prserver managed to shut down cleanly. These
issues may explain some of the hangs people have reported.

This change:

* Ensures the connection acceptance thread monitors self.quit
* We wait for the thread to exit before exitting
* We sync the database when the thread exits
* We do what the comment mentions, timeout after 30s and sync the database
  if needed. Previously, there was no timeout (the 0.5 applies to sockets,
  not the Queue object)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>