simplify federation stream traffic

This issue has been migrated from [#7549](https://github.com/matrix-org/synapse/issues/7549).

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The `federation` replication stream has four types of row:

* presence
* presence_destinations
* keyed_edus
* edus

`edus` is completely unused afaict, and `keyed_edus` is only used for typing updates. This is somewhat important, as the federation stream is not reliable (we can drop rows during restarts). Additionally, since typing notifications are typically sent to a lot of destinations, there is a lot of duplication.

Could/should we replace `keyed_edus` and `edus` with a single `typing` row type, and while we're at it remove a bunch of dead code in `FederationSender`/`FederationRemoteSendQueue`/`PerDestinationQueue` (eg `PerDestinationQueue._pending_edus` is never used)?