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/lndhub/decorators.py

@@ -15,7 +15,7 @@ def check_wallet(requires_admin=False):
             if requires_admin and key_type != "admin":
                 return jsonify({"error": True, "code": 2, "message": "insufficient permissions"})
 
-            g.wallet = get_wallet_for_key(key, key_type)
+            g.wallet = await get_wallet_for_key(key, key_type)
             if not g.wallet:
                 return jsonify({"error": True, "code": 2, "message": "insufficient permissions"})
             return await view(**kwargs)