MB-8029 Make view file sorter use new file sorter module

The view file sorter module now uses the new file_sorter.c
module instead of the module mergesort.c.

Change-Id: I8f289c09452c3f712a859237f6595e27323e20f2
Reviewed-on: http://review.couchbase.org/26190
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
MB-8029 Refactor view file merging callbacks

Now that the view file sorter uses the new file sort
module, which in turn uses the filer merger module's
callbacks, move each file merging callback into a single
common function, that deals both with view files for
initial index builds and with incremental update files.

Change-Id: Ie600c8a3d5c13803944e7bb3f1813a6af561345f
Reviewed-on: http://review.couchbase.org/26191
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
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>