Syndi: transition CrossRef and DataCite RSS feed monitoring to simple SOH pings

[servilla]

Syndi monitors CrossRef and DataCite by reading their RSS feeds and looking for relevant notifications. It should suffice to ping critical service end-points to ensure the response is as expected; otherwise, flag it as down. This approach is currently being implemented to monitor PASTA's audit service (#134).

We should transition from RSS feed monitoring to a simple service, SOH ping. This will greatly reduce the logic needed to interpret the RSS feed and remove the database necessary to store the feed content. We will still need a method to add ad-hoc messages as needed.