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>