public Cache<String, MUCRoom> getROOM_CACHE() {
        return ROOM_CACHE;
    }
This method detects rooms that we know of 'locally' (in the data structure that supports the room cache), but which
are not (no longer) in the cache.

When a cluster node crashes out of the cluster (eg: network interruption), it has been observed that the cache can
'break'. Presumably, the affected cache entry wasn't "physically" stored on the server, and as the network connection
is gone, a backup cannot be obtained either.

This method attempts to identify, remove and return rooms that are lost in these cases, which is intended to be
used (only) when processing a "cluster break" event.

@return room names that were known to the local server, but not (any more) in the clustered cache.
xmppserver/src/main/java/org/jivesoftware/openfire/muc/spi/LocalMUCRoomManager.java