check pending invoices only once on startup,
then check outgoing payments every 30 minutes,
and delete outgoing payments that return False (meaning they have failed).
also fix a bug on sparko.
a big refactor that:
- fixes some issues that might have happened (or not) with asynchronous
reactions to payments;
- paves the way to https://github.com/lnbits/lnbits/issues/121;
- uses more async/await notation which just looks nice; and
- makes it simple(r?) for one extension to modify stuff from other extensions.