optimize iterator.Next() to compare RHS of keys first
optimize iterator.Next() to compare RHS of keys first

During iteration, the theory is many keys may share the same prefix,
so the newly introduced iteratorBytesEqual() helper func optimizes by
comparing []byte's starting from the right-hand-side.

Change-Id: I87063613a8c4c146e0ab8dcf8af55148fa91cd75
Reviewed-on: http://review.couchbase.org/66097
Reviewed-by: Marty Schoch <ba9951d7d66412e2a57aea395f01176a793cf5eb@gmail.com>
Tested-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

steveyen/moss,couchbaselabs/moss,steveyen/moss