MB-8689 Add couch_set_view:group_indexed_document_ids/4

This function allows a caller to know the ID of all the documents
for which a view map function emmitted at least one Key-Value pair.

Change-Id: I95c9e0d33ed0f0626834ee85eb90cdfbc3c15624
Reviewed-on: http://review.couchbase.org/27865
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>