CBQE-2386: tuq: add doc loader for files backend

Change-Id: Ia6d94ff7dacd994715fea23aa8b9a4e34c828151
Reviewed-on: http://review.couchbase.org/40700
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>