msg/SimpleMessenger: drop msgr lock when joining a Pipe

Avoid this deadlock:

- a fault
- delay thread entry gets a fast dispatch message
 - drops delay_lock
 - calls into fast_dispatch
- reaper tries to reap the pipe
 - pipe->join()
  - delay_thread->join()
   - blocks waiting for delay_thread to exit
- delay thread / fast dispatch blocks on msgr->lock trying to mark_down

The solution is to drop the msgr lock while joining the thread.  This will
allow the join() to complete.  reaper() callers can all tolerate the
msgr->lock being dropped.

Signed-off-by: Sage Weil <sage@redhat.com>
msgr moar debug