NCBC-659: Make KetamaKeyMapper and VBucketKeyMapper Dispose HashAlgorithm after every use

Change-Id: I33ba2b21789713069f06211c1f353698ff4f771a
Reviewed-on: http://review.couchbase.org/41779
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>