MB-8029 View file sorter can now sort initial build files

With this change the couch_view_file_sorter command can now
sort temporary view files for the incremental index updates
and for the initial index builds.

Change-Id: I48cf41115081ac6dcbf6a3fa5c5377f561df3da9
Reviewed-on: http://review.couchbase.org/26192
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>