MQTTv5 client: Keep alive required to be >= 5 seconds

[Cadynum]

When using v5, the keep-alive is restricted to at least 5 seconds, but I don't find any reference to this restriction in https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901045.

rumqtt/rumqttc/src/v5/mod.rs

Lines 229 to 234 in b1c3a20

	pub fn set_keep_alive(&mut self, duration: Duration) -> &mut Self {
	assert!(duration.as_secs() >= 5, "Keep alives should be >= 5 secs");
	self.keep_alive = duration;
	self
	}

A keep alive of 0 is also permitted in v5:

A Keep Alive value of 0 has the effect of turning off the Keep Alive mechanism. If Keep Alive is 0 the Client is not obliged to send MQTT Control Packets on any particular schedule.

[swanandx]

thanks for reporting. looks like this was fixed for v4 client in #655 but not for v5.

also as discussed in mentioned PR, i believe we should change set_keep_alive to take u16 ( which we will treat as seconds ), wdyt?

[Cadynum]

I agree, a u16 fits better, as it indicates what kind of resolution is supported. I'd name it "seconds".