mon: fix Session ref leak in handle_get_version

Signed-off-by: Sage Weil <sage@inktank.com>
mon: wait for PaxosService readable in handle_get_version

We were waiting for the election to finish, but we need to *also* wait for
paxos to recover.  Being a peon or leader is not sufficient and we may
return a map that is still old.

Fixes: #7997
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1693 from ceph/wip-7997

mon: fix get_version race (more)

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>