mon/MonClient: release pending outgoing messages on shutdown

This fixes a small memory leak when we have messages queued for the mon
when we shut down.  It is harmless except for the valgrind leak check
noise that obscures real leaks.

Backport: dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #530 from ceph/wip-monc-leak

mon/MonClient: release pending outgoing messages on shutdown

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>