optimize FTSIterator.pointTo()
optimize FTSIterator.pointTo()

Thanks Sreekanth Sivasankaran for pointing out that the loop can more
optimally walk backwards when trying to find the maxQ.

Change-Id: I93c5a686021104387b1a4f9b9958916de73c28dc
Reviewed-on: http://review.couchbase.org/96978
Reviewed-by: Sreekanth Sivasankaran <fbae5d8597292a4482dedcf5071ee780420e4dc0@couchbase.com>
Tested-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

couchbase/vellum