Bug 808470 - batch changes and don't update so often when scanning

MediaDB has always tried to send change notifications in batches. This works well on the desktop, but the timeout was too short to do any good on the phone, so when the gallery app scans for new photos from the camera, the thumbnails update one at a time and there is a lot of visual jumpiness.

This PR makes the batch delay and maximum batch size configurable in media db, and then modifies gallery to use those configuration options.  Separately, it fixes the issue of flickering and the inability to zoom when viewing individual photos while a scan is in progress.

Finally, I've taken this opportunity to remove the now unused old version of MediaDB in media_db.js