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/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
     ):