mon/MonClient: fix shutdown race

mon/MonClient: fix shutdown race

While we are shutting down, we should drop all subsequent mon
messages.  Currently it is possible to finish the authentication
sequence *while* we are shutting down, such that a subsequent
attempt to authenticate succeeds.

Fix this by resetting cur_con early in the sequence, as this
makes us drop all incoming messages.

Fixes: http://tracker.ceph.com/issues/13992
Signed-off-by: Sage Weil <sage@redhat.com>