Support CSMA off

Clean packet buffer clearing when disabling the medium access. This is useful for sensors which during long periods sleeping or inactive and have no radio capabilities. 

For battery powered node, we need to keep the radio usage inside some boundaries. This caused the CSMA rtx queue to fill and loose control of the nodes. This solves the problem by clearing the queue whenever the MAC is turned off. In any case it's the correct thing to do since on the next turn on, the data is already too old.