migrate to sqlalchemy-aio.

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.

lnbits/extensions/withdraw/models.py

@@ -1,5 +1,5 @@
 from quart import url_for
-from lnurl import Lnurl, LnurlWithdrawResponse, encode as lnurl_encode
+from lnurl import Lnurl, LnurlWithdrawResponse, encode as lnurl_encode  # type: ignore
 from sqlite3 import Row
 from typing import NamedTuple
 import shortuuid  # type: ignore