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/core/__init__.py

@@ -1,5 +1,7 @@
 from quart import Blueprint
+from lnbits.db import Database
 
+db = Database("database")
 
 core_app: Blueprint = Blueprint(
     "core", __name__, template_folder="templates", static_folder="static", static_url_path="/core/static"