Proper way to decommission homeserver from federation to stop inbound traffic

This issue has been migrated from [#3765](https://github.com/matrix-org/synapse/issues/3765).

---

It would be useful to have an actual way to decommission servers. In theory, servers can leave all rooms and be done with it, but that is non-trivial and in practice servers will get federation requests for basically forever.

A possible measure would be a specific value in the SRV record, or a HTTP response that tells other servers that this server no longer participates in the federation and shouldn't be pinged again.

(kind of related to #3286, but for federation traffic instead of client traffic)