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/decorators.py

@@ -15,7 +15,7 @@ def api_check_wallet_key(key_type: str = "invoice", accept_querystring=False):
         async def wrapped_view(**kwargs):
             try:
                 key_value = request.headers.get("X-Api-Key") or request.args["api-key"]
-                g.wallet = get_wallet_for_key(key_value, key_type)
+                g.wallet = await get_wallet_for_key(key_value, key_type)
             except KeyError:
                 return (
                     jsonify({"message": "`X-Api-Key` header missing."}),
@@ -63,7 +63,9 @@ def check_user_exists(param: str = "usr"):
     def wrap(view):
         @wraps(view)
         async def wrapped_view(**kwargs):
-            g.user = get_user(request.args.get(param, type=str)) or abort(HTTPStatus.NOT_FOUND, "User  does not exist.")
+            g.user = await get_user(request.args.get(param, type=str)) or abort(
+                HTTPStatus.NOT_FOUND, "User  does not exist."
+            )
 
             if LNBITS_ALLOWED_USERS and g.user.id not in LNBITS_ALLOWED_USERS:
                 abort(HTTPStatus.UNAUTHORIZED, "User not authorized.")