MB-8159 View file merger now merges incremental files

The view merger program is now used exclusively to merge
files containing sorted operations to apply against the
view btrees. Used by the compaction retry phase.

Change-Id: Ib6d90ce41e4a234822759c0095c86d745e0fad7a
Reviewed-on: http://review.couchbase.org/26450
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>