Only transition from backfill once all items are sent

The old code allowed an upr stream to transition to in memory once
the backfill task completed even if not all of the backfill items
were sent. This change waits until all of the items are put onto
the wire.

Change-Id: I3f35c5063994743400f87d160125da373b5aaaed
Decrement backfillRemaining when we send the item

Previously we decremented backfillRemaining when the item was
queued to be sent. This change decrements backfillRemaining when
the item is actually sent.

Change-Id: I3785b15e412409481ee5defcb83517b35656e831
Calculate backfill remaining more accurately

We now have a couchstore api that will allow us to count a range
of sequence numbers to count.

Change-Id: I65e95372d7652345fa6ee4e4c0722475ac88d61e
Properly set the upr cursor offset

Change-Id: I35a40000c33bd778d09a6118f3ef0a005aaf7518
Add a stat to allow upr takeover estimates

Change-Id: I3de6c22c1b399daa69f22c0db626580a21f08810