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>
Merge pull request #1607 from ceph/wip-7997

mon: wait for quorum for MMonGetVersion

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>