fix txns for postgres

The real issue: psycopg2 operates in "implicit transaction" mode by default. Every query — even a simple SELECT — starts a transaction. So after runWithConnection runs a SELECT, the connection is "in a transaction" even though nothing needs committing. Twisted's
  ConnectionPool.runWithConnection handled this by calling rollback() after each runWithConnection call to close the implicit transaction.

  The fix should be: commit/rollback after runWithConnection completes (not before the next call), matching Twisted's behavior:

synapse/storage/native_database.py

@@ -186,7 +186,15 @@ class NativeConnectionPool:
             # Get connection inside the executor thread so that
             # _thread_local resolves to this thread's connection.
             conn = self._get_connection()
-            return func(conn, *args, **kwargs)
+            try:
+                return func(conn, *args, **kwargs)
+            finally:
+                # psycopg2 auto-starts a transaction on every query, even
+                # SELECTs. Roll back to close the implicit transaction,
+                # matching Twisted's ConnectionPool.runWithConnection
+                # which did the same to keep connections clean.
+                if self._engine.in_transaction(conn):
+                    conn.rollback()
 
         loop = asyncio.get_running_loop()
         return await loop.run_in_executor(self._executor, _inner)