Not sure why/how _handlers/_handlerList is out of sync.  This could
indicate a deeper problem.

In test_logging, the only absolutely necessary change to get working
was tcpserver.abort = 1.  But we don't want to wait infinitely
to join the threads, so give a 2.0 second timeout.

There doesn't appear to be a need for a local abort variable
in serve_until_stopped, so just use the instance member.

Note the problem is only on HEAD, not in 2.4.