[ContBk] When seeing a new vBucket UUID handle gaps from the previous one

When a new (to a given node) vBucket ID appears, unless continuous backup has just been turned on, it will be because this node has been promoted. We cannot guarantee, due to failovers, that the previous active node managed to upload all the data that was in the vBucket.

To handle this case we will "gap fill" where we check the backup location for the given vBucket ID the high seqno we have backed up, and then use the current node's SSTables to fill in. To be able to do this we need to check the previous vBucket UUID(s) using the failover table on this node.