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/example/views.py

@@ -1,7 +1,8 @@
 from quart import g, render_template
 
 from lnbits.decorators import check_user_exists, validate_uuids
-from lnbits.extensions.example import example_ext
+
+from . import example_ext
 
 
 @example_ext.route("/")