Use couchstore file API v4

Required to build against couchstore as of this change
http://review.couchbase.org/#/c/25605/2

Change-Id: I55c1ee959079abc5a23b517531b9cf9f9ea10d07
Reviewed-on: http://review.couchbase.org/25607
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>