Disable `send_federation` on main process when `federation_sender` worker connects

This issue has been migrated from [#4809](https://github.com/matrix-org/synapse/issues/4809).

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**Description:**

Would it be possible that Synapse sets `send_federation: false` internally, when a *federation_sender* connects to the replication port? The same with `enable_media_repo: false` and *media_repository*. Maybe it's possible to flip *send_federation* back, when the worker disconnects.

Maybe Synapse could also tell a second instance to not start, if another work of this kind is already running?