MB-10135 Added support for UPR_ADD_STREAM_FLAGS_DISKONLY

This flag will allow us to instantiate a UPR stream that only
reads off of disk and once finished it will end the stream.
This behavior is needed by the backup client in order to
allow the backup to get a single snapshot.

Change-Id: I6e541a9ac8c9d1064cffe4d7fba91d682ec09a4b
Reviewed-on: http://review.couchbase.org/33415
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>