ChannelDB: (trivial) add prefix to names of db methods

(and some type annotations)

This makes it clear these methods are not public.

electrum/sql_db.py

@@ -56,3 +56,6 @@ class SqlDB(Logger):
         self.conn.commit()
         self.conn.close()
         self.logger.info("SQL thread terminated")
+
+    def create_database(self):
+        raise NotImplementedError()