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/lnurlp/migrations.py

@@ -1,8 +1,8 @@
-def m001_initial(db):
+async def m001_initial(db):
     """
     Initial pay table.
     """
-    db.execute(
+    await db.execute(
         """
         CREATE TABLE IF NOT EXISTS pay_links (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
@@ -16,14 +16,14 @@ def m001_initial(db):
     )
 
 
-def m002_webhooks_and_success_actions(db):
+async def m002_webhooks_and_success_actions(db):
     """
     Webhooks and success actions.
     """
-    db.execute("ALTER TABLE pay_links ADD COLUMN webhook_url TEXT;")
-    db.execute("ALTER TABLE pay_links ADD COLUMN success_text TEXT;")
-    db.execute("ALTER TABLE pay_links ADD COLUMN success_url TEXT;")
-    db.execute(
+    await db.execute("ALTER TABLE pay_links ADD COLUMN webhook_url TEXT;")
+    await db.execute("ALTER TABLE pay_links ADD COLUMN success_text TEXT;")
+    await db.execute("ALTER TABLE pay_links ADD COLUMN success_url TEXT;")
+    await db.execute(
         """
         CREATE TABLE invoices (
             pay_link INTEGER NOT NULL REFERENCES pay_links (id),
@@ -35,14 +35,14 @@ def m002_webhooks_and_success_actions(db):
     )
 
 
-def m003_min_max_comment_fiat(db):
+async def m003_min_max_comment_fiat(db):
     """
     Support for min/max amounts, comments and fiat prices that get
     converted automatically to satoshis based on some API.
     """
-    db.execute("ALTER TABLE pay_links ADD COLUMN currency TEXT;")  # null = satoshis
-    db.execute("ALTER TABLE pay_links ADD COLUMN comment_chars INTEGER DEFAULT 0;")
-    db.execute("ALTER TABLE pay_links RENAME COLUMN amount TO min;")
-    db.execute("ALTER TABLE pay_links ADD COLUMN max INTEGER;")
-    db.execute("UPDATE pay_links SET max = min;")
-    db.execute("DROP TABLE invoices")
+    await db.execute("ALTER TABLE pay_links ADD COLUMN currency TEXT;")  # null = satoshis
+    await db.execute("ALTER TABLE pay_links ADD COLUMN comment_chars INTEGER DEFAULT 0;")
+    await db.execute("ALTER TABLE pay_links RENAME COLUMN amount TO min;")
+    await db.execute("ALTER TABLE pay_links ADD COLUMN max INTEGER;")
+    await db.execute("UPDATE pay_links SET max = min;")
+    await db.execute("DROP TABLE invoices")