Formally defined IDatabase and made a few Database methods protected

Formally defined IDatabase and made a few Database methods protected

* IDEs can now make sense of IDatabase, which is useful for
  lazy connections. The interface might also help lower coupling.
* This also updates DBConnRef, which implements IDatabase
* Some discouraged methods are left out of IDatabase.
  These methods either encourage manual query building,
  are only needed by the updater, are obscure/unused,
  or either wise encourage bad practice. DBConnRef still
  supports these if called to avoid breakage though.

Change-Id: Ia83530820f185415725c1d5f54b5172121b4938f