Rate-Limiting for set_presence changes

[matrixbot]

This issue has been migrated from #16705.

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Description:

I notice that within the last few days (probably with a new version of Element Desktop) these clients start flooding my synapse server with presence changes if there are multiple sessions.
I observed over 30+ presence requests per second from single clients, giving noticeable load on the server.
These are changes which are flipping between set_presence=online and set_presence=unavailable.
This might be a bug somewhere in Element or Synapse at the moment, but a rogue client could deliberately flood the server with presence changes which then can render it completely unusable.
There should be a rate-limiting applied for presence updates per user or per session in order to prevent this.

[jlemangarin]

Still HARD flooding the browser and the server
1.98.0