net: lwm2m: Notify when it's safe to turn RX off

According to LWM2M specification, when Queue Mode is used, the LWM2M
client should keep the reciever on for specified time after sending A
CoAP message. This commit adds a new LWM2M event,
`LWM2M_RD_CLIENT_EVENT_QUEUE_MODE_RX_OFF`, to facilitate the process by
notifying the application when it's safe to turn the receiver off.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>