Ensure Dial exits early or returns error when Transport has been
stopped, instead of blocking on the dial or ret channels. This removes
a potential goroutine leak where callers could pile up waiting
forever under heavy load.
Add select guards before send and receive, and propagate clear error
values so callers can handle shutdown gracefully.
Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>
