Reduce serialization errors in MultiWriterIdGen (#8456)

We call `_update_stream_positions_table_txn` a lot, which is an UPSERT
that can conflict in `REPEATABLE READ` isolation level. Instead of doing
a transaction consisting of a single query we may as well run it outside
of a transaction.

synapse/storage/engines/sqlite.py

@@ -17,6 +17,7 @@ import threading
 import typing
 
 from synapse.storage.engines import BaseDatabaseEngine
+from synapse.storage.types import Connection
 
 if typing.TYPE_CHECKING:
     import sqlite3  # noqa: F401
@@ -86,6 +87,7 @@ class Sqlite3Engine(BaseDatabaseEngine["sqlite3.Connection"]):
 
         db_conn.create_function("rank", 1, _rank)
         db_conn.execute("PRAGMA foreign_keys = ON;")
+        db_conn.commit()
 
     def is_deadlock(self, error):
         return False
@@ -105,6 +107,14 @@ class Sqlite3Engine(BaseDatabaseEngine["sqlite3.Connection"]):
         """
         return "%i.%i.%i" % self.module.sqlite_version_info
 
+    def in_transaction(self, conn: Connection) -> bool:
+        return conn.in_transaction  # type: ignore
+
+    def attempt_to_set_autocommit(self, conn: Connection, autocommit: bool):
+        # Twisted doesn't let us set attributes on the connections, so we can't
+        # set the connection to autocommit mode.
+        pass
+
 
 # Following functions taken from: https://github.com/coleifer/peewee