ActiveStream's initial backfill does not respect the purge-seqno

When a stream is first requested, the rollback logic checks if the start-seqno is below the purge-seqno, however by the time the backfill is actually scheduled to run, the purge-seqno could of changed (i.e. compaction finished).

• streamRequest - compare startSeqno against vb->getPurgeSeqno(), stream is OK and backfill is scheduled
• compaction finishes and sets purgeSeqno to be > than startSeqno
• backfill runs and is started from the requested point and may miss deletions which have been purged