Add a class UnpersistedEventContext to allow for the batching up of storing state groups (#14675)

* add class UnpersistedEventContext

* modify create new client event to create unpersistedeventcontexts

* persist event contexts after creation

* fix tests to persist unpersisted event contexts

* cleanup

* misc lints + cleanup

* changelog + fix comments

* lints

* fix batch insertion?

* reduce redundant calculation

* add unpersisted event classes

* rework compute_event_context, split into function that returns unpersisted event context and then persists it

* use calculate_context_info to create unpersisted event contexts

* update typing

* $%#^&*

* black

* fix comments and consolidate classes, use attr.s for class

* requested changes

* lint

* requested changes

* requested changes

* refactor to be stupidly explicit

* clearer renaming and flow

* make partial state non-optional

* update docstrings

---------

Co-authored-by: Erik Johnston <erik@matrix.org>

tests/test_utils/event_injection.py

@@ -92,8 +92,13 @@ async def create_event(
     builder = hs.get_event_builder_factory().for_room_version(
         KNOWN_ROOM_VERSIONS[room_version], kwargs
     )
-    event, context = await hs.get_event_creation_handler().create_new_client_event(
+    (
+        event,
+        unpersisted_context,
+    ) = await hs.get_event_creation_handler().create_new_client_event(
         builder, prev_event_ids=prev_event_ids
     )
 
+    context = await unpersisted_context.persist(event)
+
     return event, context