Refactor to ensure we call check_consistency (#9470)

The idea here is to stop people forgetting to call `check_consistency`. Folks can still just pass in `None` to the new args in `build_sequence_generator`, but hopefully they won't.

synapse/storage/database.py

@@ -49,7 +49,6 @@ from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage.background_updates import BackgroundUpdater
 from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine, Sqlite3Engine
 from synapse.storage.types import Connection, Cursor
-from synapse.storage.util.sequence import build_sequence_generator
 from synapse.types import Collection
 
 # python 3 does not have a maximum int value
@@ -381,7 +380,10 @@ class DatabasePool:
     _TXN_ID = 0
 
     def __init__(
-        self, hs, database_config: DatabaseConnectionConfig, engine: BaseDatabaseEngine
+        self,
+        hs,
+        database_config: DatabaseConnectionConfig,
+        engine: BaseDatabaseEngine,
     ):
         self.hs = hs
         self._clock = hs.get_clock()
@@ -420,16 +422,6 @@ class DatabasePool:
                 self._check_safe_to_upsert,
             )
 
-        # We define this sequence here so that it can be referenced from both
-        # the DataStore and PersistEventStore.
-        def get_chain_id_txn(txn):
-            txn.execute("SELECT COALESCE(max(chain_id), 0) FROM event_auth_chains")
-            return txn.fetchone()[0]
-
-        self.event_chain_id_gen = build_sequence_generator(
-            engine, get_chain_id_txn, "event_auth_chain_id"
-        )
-
     def is_running(self) -> bool:
         """Is the database pool currently running"""
         return self._db_pool.running