mon: prevent new sessions during shutdown

[liewegas]

From shutdown() we set STATE_SHUTDOWN and then call remove_all_sessions().
ms_handle_accept() is the only caller of add_session, so verifying that
we aren't shutting down (while under the session_map_lock) is sufficient
to prevent any new sessions from being added.

Fixes: https://tracker.ceph.com/issues/39150

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