Sending typing EDUs causes high load in the federationapi

*This issue was originally created by [**@S7evinK**](https://github.com/S7evinK) at .*

More as a reminder.
Possibly related, as this can degrade QoS - #1622 and maybe #2079

### Background information

- **Dendrite version or git SHA**: 5106cc807cf22a95420b24f6bfdd5c9ac8aa06de
- **Monolith or Polylith?**: polylith
- **SQLite3 or Postgres?**: postgres
- **Running in Docker?**: more or less
- **`go version`**: 1.17.x
- **Client used (if applicable)**: Element Web 1.10.2-rc.2

### Description

- **What** is the problem: High CPU load when sending typing EDUs in crowded room
- **Who** is affected: me (but would affect other users)
- **How** is this bug manifesting: long delay in sending messages, as the fedapi is busy
- **When** did this first appear: today, after wiping my database

### Steps to reproduce

- join #dendrite:matrix.org
- start typing a message
- wait 30s (typing timeout)
- start typing again

Several of those log entries:
```
time="2022-02-11T23:22:20.444381367Z" level=info msg="Sending EDU event" destinations=888 edu_type=m.typing
```

Disabling `Send typing notifications` in Element Web helps in this case, but stuff like read markers could probably result in the same behavior on busy servers/rooms.