Incorrect max_idle_timeout negotiation.

[mstyura]

Steps to reproduce

1. Configure client side with custom transport parameters by overriding max_idle_timeout to Duration::ZERO
2. Connect to quinn-based remote server which configured max_idle_timeout some some value, e.g. 180 seconds. During handshake client side will omit max_idle_timeout parameter inside TLS handshake to server which seems to correspond to this code:

   quinn/quinn-proto/src/transport_parameters.rs

   Lines 36 to 37 in 15a4dce

   	/// Milliseconds, disabled if zero
   	max_idle_timeout(0x0001) = 0,

Actual result

Quinn on server side will negotiate idle_timeout to be 180 seconds because not value from client received, but client side connection will actually compute idle_timeout to be 0:

quinn/quinn-proto/src/connection/mod.rs

Lines 3293 to 3298 in 15a4dce

	self.idle_timeout = match (self.config.max_idle_timeout, params.max_idle_timeout) {
	(None, VarInt(0)) => None,
	(None, x) => Some(x),
	(Some(x), VarInt(0)) => Some(x),
	(Some(x), y) => Some(cmp::min(x, y)),
	};

effectively falling back to 3 * PTO timeout:

quinn/quinn-proto/src/connection/mod.rs

Lines 1845 to 1856 in 15a4dce

	fn reset_idle_timeout(&mut self, now: Instant, space: SpaceId) {
	let timeout = match self.idle_timeout {
	None => return,
	Some(x) => Duration::from_millis(x.0),
	};
	if self.state.is_closed() {
	self.timers.stop(Timer::Idle);
	return;
	}
	let dt = cmp::max(timeout, 3 * self.pto(space));
	self.timers.set(Timer::Idle, now + dt);
	}

Due to low effective idle timeout connection is frequently locally closed and need to be re-established.

Expected result

I'd expect that both sides agree on the same value of max_idle_timeout so in case local side configured with 0 timeout and peer side have non-zero idle timeout configured both endpoints will use same non-zero idle timeout.

This expectation seems to match the RFC where 0 value is threaten as disabled timeout according to section 18.2 which already respected while writing transport parameters to wire format.

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.

And according to section 10.1 during negotiation in case of only one side have enabled max_idle_timeout both endpoints must negotiate to the value provided by other side.

Each endpoint advertises a max_idle_timeout, but the effective value at an endpoint is computed as the minimum of the two advertised values (or the sole advertised value, if only one endpoint advertises a non-zero value).