mon/MonClient: make get_mon_log_message() atomic

mon/MonClient: make get_mon_log_message() atomic

* LogClient: move reset_session() into get_mon_log_message() and add a
  "flush" param to the latter. so it can get_mon_log_message()
  atomically. otherwise another call changing the log queue could sneak
  in between reset_session() and get_mon_log_message().
* MonClient: add a "flush" param to do_send() so we can reset the
  LogClient session once we are connected to a monitor.

Fixes: http://tracker.ceph.com/issues/19427
Signed-off-by: Kefu Chai <kchai@redhat.com>