fix(iroh-net): delays of non-stun probes for subsequent relays (#2445)

## Description

The idea of the probe plan is that first the STUN probes happen, if
those don't work we add the other probes.  However when there are
multiple relay servers we accidentally started the subsequent probes
for all but the first relay server too late.

This makes sure to globally record when the last STUN probe was sent
and re-uses this value for all the relay servers which are probed.

## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->

## Notes & open questions

Fixes #2444

See #2221 for flaky tests.

## Change checklist

- [x] Self-review.
- ~~[ ] Documentation updates if relevant.~~
- [x] Tests if relevant.
- ~~[ ] All breaking changes documented.~~