Allow specification of start/end seqno ids when doing backfills

This will allow upr clients to specify a range of data they they
want to receive when doing a backfill. Note that we can likely
refactor the backfill logic in the future to make it simpler.

Change-Id: Ib95d0693838cd405bbc532f50cf7386cc94ffaac
Reviewed-on: http://review.couchbase.org/30765
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>