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

1 files changed, 3 insertions, 1 deletions

diff --git a/tests/handlers/test_federation_event.py b/tests/handlers/test_federation_event.py
index 489ba57736..e64b28f28b 100644
--- a/tests/handlers/test_federation_event.py
+++ b/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: