Reduce the amount of room members we pull out when calculating push actions

[erikjohnston]

When we calculate push actions for an event we fetch all users that were in the room at the time:

synapse/synapse/push/bulk_push_rule_evaluator.py

Line 200 in ef86cf3

room_members = await self.store.get_joined_users_from_context(event, context)

However, we only use the result in two places:

1. For getting the count of the number of users in the room (for evaluating push rules that are based on a count)
2. Getting the display name of local user who has push enabled

Thus for small servers that only have one or two users in the room, fetching the full state is overkill and causes increased memory and DB usage.

We separately do another state query to find out who should be pushed in the room, at:

synapse/synapse/push/bulk_push_rule_evaluator.py

Line 418 in ef86cf3

current_state_ids = await context.get_current_state_ids()

We should figure out a way to not do this.