Online state incorrect

[DutchmanNL]

If connection to printer is lost, MQTT does not send a proper "end" event, due to that online state keeps on "true"

solution:

Implement watchdog to:

• validate timestamp of last message by schedule
• if timestamp is > (configurable value) printer is "considered offline"
• pushily event will be send to printer to verify if connection is lost
• if no answer is received within 5 second, connection state will be set to FALSE
• watched keeps running by schedule to ensure online state handling at reconnection

[DutchmanNL]

relates #52