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/diagonalley/migrations.py

@@ -1,8 +1,8 @@
-def m001_initial(db):
+async def m001_initial(db):
     """
     Initial products table.
     """
-    db.execute(
+    await db.execute(
         """
         CREATE TABLE IF NOT EXISTS products (
             id TEXT PRIMARY KEY,
@@ -20,7 +20,7 @@ def m001_initial(db):
     """
     Initial indexers table.
     """
-    db.execute(
+    await db.execute(
         """
         CREATE TABLE IF NOT EXISTS indexers (
             id TEXT PRIMARY KEY,
@@ -41,7 +41,7 @@ def m001_initial(db):
     """
     Initial orders table.
     """
-    db.execute(
+    await db.execute(
         """
         CREATE TABLE IF NOT EXISTS orders (
             id TEXT PRIMARY KEY,