Don't needlessly batch in `add_event_to_cache` (#10784)

We've already batched up the events previously, and assume in other
places in the events.py file that we have. Removing this makes it easier
to adjust the batch sizes in one place.

changelog.d/10784.misc

@@ -0,0 +1 @@
+Minor speed ups when joining large rooms over federation.

synapse/storage/databases/main/events.py

@@ -1547,11 +1547,10 @@ class PersistEventsStore:
         to_prefill = []
 
         rows = []
-        N = 200
-        for i in range(0, len(events_and_contexts), N):
-            ev_map = {e[0].event_id: e[0] for e in events_and_contexts[i : i + N]}
+
+        ev_map = {e.event_id: e for e, _ in events_and_contexts}
         if not ev_map:
-                break
+            return
 
         sql = (
             "SELECT "
@@ -1573,9 +1572,7 @@ class PersistEventsStore:
         for row in rows:
             event = ev_map[row["event_id"]]
             if not row["rejects"] and not row["redacts"]:
-                    to_prefill.append(
-                        _EventCacheEntry(event=event, redacted_event=None)
-                    )
+                to_prefill.append(_EventCacheEntry(event=event, redacted_event=None))
 
         def prefill():
             for cache_entry in to_prefill: