Use ConnMap in backfill task

This will allow us to specify different behavior based on whether
or not the connection is for tap or upr.

Change-Id: I6f799e68e3d62902e913919126707cb482bc454b
Reviewed-on: http://review.couchbase.org/30764
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
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>