Version uploaded while another version is PUBLIC_WAITING goes to wrong queue (bug 868526)

https://bugzilla.mozilla.org/show_bug.cgi?id=868526

If an app is in PUBLIC_WAITING state, then:
- Addon.get_version() should return the current PUBLIC_WAITING version
- Webapp.update_status() shouldn't erase PUBLIC_WAITING when there is a new version
- When new versions are uploaded for that app, it should go in the update queue