state: per-goroutine deadline in HA probe cycle

`time.After(ProbeTimeout)` returned a single channel shared by every
probe goroutine in the cycle. Only the first goroutine to receive the
deadline tick drains the channel; any other goroutine still waiting on
its `responseCh` is then stuck forever, `wg.Wait()` never returns, and
the scheduler loop in `app.go` stalls on the next tick. The condition
fires whenever two or more nodes time out in the same cycle — common
under cable-pull where IsOnline lags reality and both routers stay in
the candidate set as half-open TCP.

Move the timer inside each goroutine so every probe has its own
deadline.

Updates #3234