NCBC-719: Ensure operations use FMap if available

This commit checks to see if a VBucket forward map is available. If it is
the index of the VBucket Forward Map will be used, otherwise the VBucket's
index is used.

Change-Id: I817f818058e49884bc87dc85d0935654a18d3e62
Reviewed-on: http://review.couchbase.org/43589
Reviewed-by: Simon Baslé <simon@couchbase.com>
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>