New option tombstone_expiry locks recent room creation after destroy (#2546)

New option tombstone_expiry locks recent room creation after destroy (#2546)

Setting the new mod_muc option tombstone_expiry to a positive integer
will make that any room destroyed gets replaced with a room tombstone.
That tombstone cannot be joined, so it blocks accessing the old room JID
until the expiry seconds have passed.
The default value is 0 seconds, so tombstones are not created.