MB-7629 Rename functions from plural to singular

These functions operate on a single reduction structure,
therefore their name must be singular and not plural.

Change-Id: I286d916a160864f6d01f572c2f69c8fbab9b2b83
Reviewed-on: http://review.couchbase.org/26782
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>