Remove backfill callbacks from the ConnMap class

UPR will have it's own seperate backfilling code so we no longer
need to use the callbacks that Tap conenctions use when they do
backfill. Having our own UPR backfilling logic should also lower
lock contention since we will lock based on the Stream as opposed
to the Connection.

Change-Id: I6786b0f1397d94b6ed126a7b62afb30eaceae53a
Reviewed-on: http://review.couchbase.org/31461
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>