controller should set agv state unkonwn when missing state msg

[vincentknw]

User story / use case

when not recieve state msg from a agv to long; controller should set agv state UNKNOWN。

reason

now controller still remain last state IDLE，agv cannot send disconnect msg sometime.

[swltr]

@vincentknw The VDA5050 specification describes a clear indication of whether or not a vehicle is available in the form of the connection topic and the vehicle's last will:

• When the vehicle disconnects gracefully, it is supposed to publish OFFLINE as its connectionState.
• For cases in which the vehicle's connection to the MQTT broker is interrupted unexpectedly, the vehicle is supposed to register a last will that publishes CONNECTIONBROKEN as its connectionState.

As long as the vehicle adheres to the specification, checking for incoming state messages is neither required by the specification nor necessary.