Ember key table should be cleared when device leaves

[cdjackson]

When a device leaves the network, or when a device joins the network, we need to ensure that the Ember key table is cleared of any device specific link keys for this device. Failure to do this could mean that an old link key is used, and if the device will use the standard HA link key it will be unable to join the network.

This requires some thought as to how and when to do this. If the device is using install codes, this is not a problem, but if the device is using the standard HA TC link key, but setting a device specific link key after joining, then this needs to be cleared before the device will be able to leave and successfully rejoin the network.

This clearing of the key needs to be device specific so as not to remove other link keys that are in use in the network.

This is partially resolved in #700 where the key table is cleared when the network is reinitialised.