Refactor EventContext (#12689)

Refactor how the `EventContext` class works, with the intention of reducing the amount of state we fetch from the DB during event processing.

The idea here is to get rid of the cached `current_state_ids` and `prev_state_ids` that live in the `EventContext`, and instead defer straight to the database (and its caching). 

One change that may have a noticeable effect is that we now no longer prefill the `get_current_state_ids` cache on a state change. However, that query is relatively light, since its just a case of reading a table from the DB (unlike fetching state at an event which is more heavyweight). For deployments with workers this cache isn't even used.


Part of #12684

tests/handlers/test_federation_event.py

@@ -148,7 +148,9 @@ class FederationEventHandlerTests(unittest.FederatingHomeserverTestCase):
             prev_event.internal_metadata.outlier = True
             persistence = self.hs.get_storage().persistence
             self.get_success(
-                persistence.persist_event(prev_event, EventContext.for_outlier())
+                persistence.persist_event(
+                    prev_event, EventContext.for_outlier(self.hs.get_storage())
+                )
             )
         else:
 

tests/storage/test_event_chain.py

@@ -393,7 +393,7 @@ class EventChainStoreTestCase(HomeserverTestCase):
             # We need to persist the events to the events and state_events
             # tables.
             persist_events_store._store_event_txn(
-                txn, [(e, EventContext()) for e in events]
+                txn, [(e, EventContext(self.hs.get_storage())) for e in events]
             )
 
             # Actually call the function that calculates the auth chain stuff.

tests/test_state.py

@@ -88,6 +88,9 @@ class _DummyStore:
 
         return groups
 
+    async def get_state_ids_for_group(self, state_group):
+        return self._group_to_state[state_group]
+
     async def store_state_group(
         self, event_id, room_id, prev_group, delta_ids, current_state_ids
     ):

tests/test_visibility.py

@@ -234,7 +234,9 @@ class FilterEventsForServerTestCase(unittest.HomeserverTestCase):
         event = self.get_success(builder.build(prev_event_ids=[], auth_event_ids=[]))
         event.internal_metadata.outlier = True
         self.get_success(
-            self.storage.persistence.persist_event(event, EventContext.for_outlier())
+            self.storage.persistence.persist_event(
+                event, EventContext.for_outlier(self.storage)
+            )
         )
         return event