massive storm of presence EDUs when joining a large room

[matrixbot]

This issue has been migrated from #3120.

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after you do your send_join to a room, all the servers in the room seem to immediately send you presence updates. Since this will often be the first contact you get from those servers, we then end up doing hundreds of server key requests, all of which end up starving out the actual join request - contribuiting to the join itself taking ages.