MB-8029 Add program to merge view files

This is going to be invoked by CouchDB in order to merge a
group of view files. Used for indexing and index compaction
retry phase.

Change-Id: Ibb3ff327d96eb6dee61dbd696415d0e95a8e786a
Reviewed-on: http://review.couchbase.org/25795
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>