Add feed callback and skip_writeback option in file sorter

For the file sorter, we write out intermediate sorted files
to disk and perform a k-way merge to produce final sorted
records file. The final records file is consumed by index
builder to build btree indexes. But, ideally we could skip
writing the final result of k-way merge to disk if we could
pipeline the results coming out of k-way merge directly into
index builder. A callback mechanism has been added to the file
sorter/merger to call a feed_record method during the final
phase of file merger. Also added a flag to skip writing final
sorted results to a file.

The consumer of file sorter can save a lot of disk IO
by avoiding unnecessary write-read of final sort results.

Change-Id: If9b15092bdc7242772981859344f4f88dec45998
Reviewed-on: http://review.couchbase.org/32659
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>