If you pass in a "since" timestamp for EnkiAPI.aviailability(), the timestamp will be ignored and the circulation manager will ask Enki for information on every book in the collection.

I can't just fix this because the API method we're invoking, ListAPI.getAllEntries, isn't documented in http://enkilibrary.org/documentation/API/index.html, so I'm not sure how the timestamp should be passed in or whether a timestamp is supported at all.

This is blocking https://jira.nypl.org/browse/SIMPLY-687.