MB-22451: Ensure isBackfillTaskRunning is correctly set

MB-22451: Ensure isBackfillTaskRunning is correctly set

In ActiveStream::completeBackfill if in STREAM_BACKFILLING state and
pendingBackfill is true then we will schedule another backfill.  This
will cause isBackfillTaskRunning to be set to true.  The flag should
remain true on exit of the completeBackfill function.

Change-Id: If8219a7f87b65af46d37a800eebf2257917cc555
Reviewed-on: http://review.couchbase.org/73310
Well-Formed: Build Bot <build@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>