This is cleanup from the introduction to rosmar.

- rosmar.Collection and Collection already satisified the DataStoreName
interface, so putting it as part of DataStore removes need for
AsDataStoreName checks.
- remove GetFeedType() checks since it will always return DCP

- Remove StartTapFeed from leaky bucket because it was not being used.
  TestStopChangeCache was only running with xattrs disabled, but the
  test would pass even if the events arrived out of order, so the leaky
  bucket tap code was not being executed.
- Reflect in documentation that DCP feed is always used.