Collections: backfillRemaining accounting is incorrect if deleted collections exist

As seen while investigating MB-35594, the ActiveStream::backfillRemaining count is inaccurate when there have been collections which have been deleted. As a consequence ns_server could end up having incorrect progress (and maybe even hanging?) during rebalance.

The issue is how backfill remaining is calculated (and updated) - it is initially set to the number of items returned from initScanContext - which is the number of items in the B-Tree. It is then decremented by one each time an item it processed from disk.

However, in the case of deleted collections, CouchKVStore::recordDbDump skips mutations for collections which have been logically deleted, never informing ep-engine of them. As such backfillRemaining isn't decremented for these skipped items.