Bug 831701 - Finish active updates unless the user swtiches to a newer one. Remove stale updates. r=rstrong

Once the user starts to download an update, stick to that update until the user cancels the download and does a "Check Now".

If a newer update is started and a stale update is detected, remove the stale update.

If a download fails due an update being in the applied state, prompt the user to apply it.

If an update was fully downloaded, but interrupted during verification, then don't redownload.