Add table for ignored users to make calculating push rules faster for large rooms.

[erikjohnston]

For large rooms we spend quite a lot of time figuring out if anyone ignores the user:

synapse/synapse/push/bulk_push_rule_evaluator.py

Lines 206 to 213 in 5d34f40

	for uid, rules in rules_by_user.items():
	if event.sender == uid:
	continue
	if not event.is_state():
	is_ignored = await self.store.is_ignored_by(event.sender, uid)
	if is_ignored:
	continue

which will look up the account data for each uid user. We can make this way more efficient by having a separate table for ignored users (updated when the account data is updated), which would allow us to do a single query to figure out everyone who ignores event.sender.

(The table should be quite small since few people are ignored)

[clokep]

This should be pretty straightforward to do for new data, but we would need to handle migrated current data into this form. We'll also need to figure out what to do with invalid account data, but likely we can just say that no one is ignored in that case.

[erikjohnston]

We'll also need to figure out what to do with invalid account data, but likely we can just say that no one is ignored in that case.

That's what we do currently fwiw