mon/MonClient: mark_down connection on shutdown

Signed-off-by: Sage Weil <sage@inktank.com>
mon/MonClient: mark_down during get_monmap_privately() shutdown

We explicitly mark_down() and clear cur_con when shutting down; do the same
for get_monmap_privately() to ensure that the reset event doesn't make us
do something silly (like, in this case, call _reopen_session() again).

Signed-off-by: Sage Weil <sage@inktank.com>