MB-8241: Refactor couch_set_view_updater module

Refactor all MapReduce View specific code out of the
couch_set_view_updater module into a new mapreduce_view
module.

Change-Id: I48fcf252d81bd5ed97400783829730d0982577a1
Reviewed-on: http://review.couchbase.org/25932
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>