Improve smartSleep: add a "no more commands" message to the gateway

[abmantis]

Currently, smartSleep() simply waits a specified amount of time for any command from the gateway.
Instead, after sending the heartbeat, the node could wait (with a timeout value) for a "no more commands" message from the gateway and sleep after receiving that. This would reduce the waiting time before sleeping.
Another improvement would be having a "request commands" from the node instead of the heartbeat.

So, on the node:

• Send "request commands"
• Wait until "no more commands" is received
• Sleep

On the gateway:

• "request commands" is received from a node
• Send commands to node
• Send "no more commands" message.

[MartinHjelmare]

This is good. I assume the gateway in the above description means the controller.

[tekka007]

see #722, use dedicated messages