1:1 redactions aren't great for server health during large spam waves

[turt2live]

The cycle:

1. Receive spam event
2. Redact event
3. Repeat

This means if 10k spam events are received, we send out 10k redactions plus the number of bans from other sources.

matrix-org/matrix-spec-proposals#4293 is considered the immediate-term fix for this.