MB-8159 Use couch_view_file_merger for view compaction

The compaction retry phase now uses the couch_view_file_merger
program from couchstore.

Change-Id: Id01be9ca0ca4ea79e58e2e12c714f885b9070516
Reviewed-on: http://review.couchbase.org/26452
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>