remove most uses of ensureStore

Created by: camdencheek

This removes most uses of ensureStore methods on our stores. These were necessary when we had global stores to make sure that we initialized the stores after dbconn.Global was initialized. Now that the store types are private and there are no global stores for these types, they must be created through their constructor, which requires a valid dbutil.DB, making ensureStore a no-op.

The only store that we could not safely remove ensureStore from is repoStore because GlobalRepos still exists and depends on this behavior.