MB-8029 Add file_merger module

This module merges a group of sorted files into a single
sorted file.

Merging files is used for indexing and index compaction
retry phase.

Change-Id: I13279ac0a5f661857c18c962e0f00bf2e7c11f79
Reviewed-on: http://review.couchbase.org/25793
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>