MB-8029 Add view file merger module

This module is meant to be used for indexing and index
compaction retry phase.

Change-Id: I6b15b1c333afb0ec254aaad088efcf6df6bedd68
Reviewed-on: http://review.couchbase.org/25794
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>