Make push rules use proper structures. (#13522)

This improves load times for push rules:

| Version              | Time per user | Time for 1k users | 
| -------------------- | ------------- | ----------------- |
| Before               |       138 µs  |             138ms |
| Now (with custom)    |       2.11 µs |            2.11ms |
| Now (without custom) |       49.7 ns |           0.05 ms |

This therefore has a large impact on send times for rooms
with large numbers of local users in the room.

Author: erikjohnston

changelog.d/13522.misc

@@ -0,0 +1 @@
+Improve performance of sending messages in rooms with thousands of local users.