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>
(cherry picked from commit 309569a6d0b7df263654b7f3f15b910a72f2918d)