MB-11325: Fix off by one error when scheduling backfills

When we register a checkpoint cursor the seqno returned is the
first seqno that will be received for that cursor. As a result we
should backfill up to one less than where the cursor is scheduled.

Change-Id: I3de5c8244f68fbb764f856877a04c2f29fb1f4be
Reviewed-on: http://review.couchbase.org/38242
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>