Sensor improvements

[ripplebiz]

Adds region_map support to the sensor example.

Adds new binary request types: REQ_TYPE_SUBSCRIBE, REQ_TYPE_UNSUBSCRIBE
These enable a permitted client node to subscribe to push telemetry, with the following guard rails:

• a fallback region scope must be resolvable (either via the SUBSCRIBE request scope, or sensor node's default scope)
• subscription has a timeout specified (and an upper limit), so re-subscribes are necessary
• subscriber must specify which telemetry channels/types to monitor and the minimum deltas which will trigger a push (passed as an LPP-encoded byte array in the SUBSCRIBE request)

[IoTThinks]

Wow, I like it.
I'm working with sensor example recently.

So we have "subscribe model" now?