mon: PaxosService: can be readable even if proposing

As long as we have a stable version in memory that is lower or equal to
the version we want.

Fixes: #9321
Fixes: #9322

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
Merge pull request #2945 from ceph/wip-9321

mon: PaxosService: can be readable even if proposing

Reviewed-by: Sage Weil <sage@redhat.com>