Mark at least one parent hash unused if message fails to send

[GallagherCommaJack]

The bug that just won't die - now we're running into issues because we might end up reusing a (key,nonce) pair due to network outages. This happens because when we mark the pending message as used, we end up having no unused keys in the store.
Should be fairly easy to fix without even touching chainmail, but at this point I'm noticing enough footguns in that API that I'm likely to change it as well.

[GallagherCommaJack]

Fixed in chainmail by generating a new nonce for every message. The more thorough API change is still probably a good idea, but not strictly necessary.