net: lwm2m: Add transmission state indicator to RX as well

Refactored the socket state indication into its own function
that checks the state of TX queues as well as number of
pending CoAP responses.
Check the state after receiving a packet, as it might
have been a last Ack packet we have been waiting.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>