MB-8241: Refactor couch_set_view_group module

Refactor all MapReduce View specific code out of the
couch_set_view_group module into the mapreduce_view
module.

Change-Id: I8bf618bc7cccdfd277a68ff9a1e5c906919d0f30
Reviewed-on: http://review.couchbase.org/26123
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>