iroh-relay: Investigate setting lower `PER_CLIENT_SEND_QUEUE_DEPTH`

[matheus23]

This comment isn't actionable. But 🤯
Used to be 32 in the tailscale code by the looks of the old comment. I suspect this was turned up at some point because it make a backpressure problem go away for a particular example. But we've actually improved backpressure since. Could we create an issue to investigate this and see if this number can be turned down?

Originally posted by @flub in #3331 (comment)