support RFC9000 max_idle_timeout=0

[marcblanchet]

RFC9000
" max_idle_timeout (0x01): The maximum idle timeout is a value in
milliseconds that is encoded as an integer; see (Section 10.1).
Idle timeout is disabled when both endpoints omit this transport
parameter or specify a value of 0."

This is currently unsupported. It may be very useful for high latency or delay networks where, instead of increasing the max_idle_timeout to very large values and having all sorts of side effects, it may be more useful to support this feature.

[martinthomson]

This is really a feature request for an unbounded idle timeout. That's a reasonable request. I'd be happy to review a change where the configuration was changed and we disabled the idle timeout as a result. The server side of this might have some surprises for us, but I don't foresee many problems.

Implementation-wise, the biggest question is whether we recognize Duration::from_xxx(0)/Duration::new(0, 0) as special and disable the timeout, or whether we move to Option<Duration>. The former might be easier.