mon/HealthMonitor: do not send MMonHealthChecks to pre-luminous mon
During upgrade, we start doing the new health check code, but if we are
not the leader we can't send the new MMonHealthChecks message to the leader
or else it will crash.
This fix goes directly into the luminous branch since post-luminous code
does not have this compatibility issue.
Fixes: http://tracker.ceph.com/issues/24481 Signed-off-by: Sage Weil <sage@redhat.com>