Don't hold onto full state in state cache

synapse/state/__init__.py

@@ -321,6 +321,10 @@ async def compute_event_context(
                     )
                 )
                 entry.state_group = state_group_before_event
+
+                # We null out the state here so that the state cache doesn't
+                # hold on to it indefinitely.
+                entry._state = None
             else:
                 state_group_before_event = entry.state_group