Allow multiple backfills from an ephemeral vBucket

Ephemeral vBuckets currently only allow a single range read over the sequence list; this prevents multiple DCP consumers backfilling concurrently.

With more than one replica, or while moving/building actives/replicas during a rebalance that this may be a bottleneck. Considering that there may also be streams for the indexer or projector/XDCR, and that streams which are "up to date" and streaming from memory can still be forced back to backfill by cursor dropping, there may be reasonably high delays for some streams.

Some work in-progress for MB-36285 includes refactors laying the ground work for this MB.