For backwards compat, make state_groups.id have a type of int, not varchar

synapse/storage/schema/full_schemas/16/state.sql

@@ -14,7 +14,7 @@
  */
 
 CREATE TABLE IF NOT EXISTS state_groups(
-    id VARCHAR(20) PRIMARY KEY,
+    id BIGINT UNSIGNED PRIMARY KEY,
     room_id VARCHAR(150) NOT NULL,
     event_id VARCHAR(150) NOT NULL
 );

synapse/storage/state.py

@@ -93,7 +93,7 @@ class StateStore(SQLBaseStore):
 
         state_group = context.state_group
         if not state_group:
-            state_group = _make_group_id(self._clock)
+            state_group = self._state_groups_id_gen.get_next_txn(txn)
             self._simple_insert_txn(
                 txn,
                 table="state_groups",