Make sort_spatial_kvs_file suitable as record file sorter

When building the intitial index, the records are written into a
single file that then gets sorted. This commit makes the
`sort_spatial_kvs_file` function comply to the API of the record
file sorted callback.

Change-Id: I6898db1eb05d108102858119a96f449d83ca0242
Reviewed-on: http://review.couchbase.org/39634
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>