v3: Merge PingreqChannelHandler into MQTTChannelHandler

The Pingreq channel handler schedules regular PINGs to be sent to the server and waits for the responding PINGRESP. If none is received then it closes the connection.

This should also be merged into the MQTTChannelHandler mentioned in #179 and instead of using the client to write PINGS it would use the task handler code in MQTTChannelHandler to sent pings.