abzsubmit support for checking against server before running essentia

[ianmcorvidae]

The server doesn't currently keep everything it's submitted, just one thing for each MBID -- so it'll only make a change if you're replacing lossy with lossless or submitting something new. Might speed things up to let abzsubmit parse the tags, check, and then only run essentia/resubmit if it'll get used.

Might benefit from server support for this check as well, of course.