MB-8029 Use native file merger for index builds

Use the couch_view_file_merger, written in C and in the
couchstore repository, to merge index files.

Sample test:

Building an index for a design document with 4 views, from
one of the evperf tests, for a dataset of 6.5M documents:

Before this change:

Indexing time:   14m54.482s
Final file size: 758Mb

After this change:

Indexing time:   12m12.986s
Final file size: 758Mb

Change-Id: I56f2143768251041ed6ff684dbe8074dc601b2f2
Reviewed-on: http://review.couchbase.org/25798
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>