MB-12179: Allow backfills to be intervleaved

Provides the structure to pause and resume backfills.

Change-Id: I96df9076885878eea46eff32213b871c7ba3e61d
Reviewed-on: http://review.couchbase.org/42315
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>