refactor(iroh-net): Don't send pings over the actor channel (#1678)

## Description

Small refactor in iroh-net: Do not send disco messages (pings and pongs)
over the main actor channel. Instead, either send them right away if the
request comes from a poll function, or use a new, small, dedicated task
that just sends out the disco messages over UDP. For Derp we drop
messages anyway if the channel is full, so no need to pass over the main
actor.


## Notes & open questions

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## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [ ] Tests if relevant.

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Co-authored-by: Friedel Ziegelmayer <[email protected]>