osd: fix watch_lock vs map_lock ordering

osd: fix watch_lock vs map_lock ordering

watch_lock is inside map_lock (and pg->lock), which means we need to
drop it to take pg->lock here.  That means verifying in
handle_watch_timeout that we haven't raced with another thread canceling
the timeout event, which would be indicated by

 - the entity not appearing in unconnected_watchers
 - the entity having a different (presumably newer) expire time

Fixes: #2103
Signed-off-by: Sage Weil <sage.weil@dreamhost.com>
Reviewed-by: Samuel Just <samuel.just@dreamhost.com>