mon: wait for quorum for MMonGetVersion

We should not respond to checks for map versions when we are in the
probing or electing states or else clients will get incorrect results when
they ask what the latest map version is.

Fixes: #7997
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 67fd4218d306c0d2c8f0a855a2e5bf18fa1d659e)