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/__main__.py

@@ -1,13 +1,17 @@
-from .app import create_app
-from .commands import migrate_databases, transpile_scss, bundle_vendored
-from .settings import LNBITS_SITE_TITLE, SERVICE_FEE, DEBUG, LNBITS_DATA_FOLDER, WALLET, LNBITS_COMMIT
+import trio  # type: ignore
 
-migrate_databases()
+from .commands import migrate_databases, transpile_scss, bundle_vendored
+
+trio.run(migrate_databases)
 transpile_scss()
 bundle_vendored()
 
+from .app import create_app
+
 app = create_app()
 
+from .settings import LNBITS_SITE_TITLE, SERVICE_FEE, DEBUG, LNBITS_DATA_FOLDER, WALLET, LNBITS_COMMIT
+
 print(
     f"""Starting LNbits with
   - git version: {LNBITS_COMMIT}