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.
instead of multiple keys/macaroons with different permissions we request only one.
if someone wants to use lnbits with an invoice macaroon they're free to do it and
we will just fail on 'pay' methods, as before.
this also grandfathers the previous environment variables names so everything keeps
working without people having to change their setups.
in the meantime some bugs with lntxbot and c-lightning were fixed and the `requests`
dependency was eliminated because I can't organize myself into meaningful chunks of
changes.
