net: lwm2m: DTLS session cache enable and queue mode update

Enabled DTLS session cache for support session resume.
Fixed LwM2M queue mode for close connection and reconnect automatically.
Re-connect will do Registration update before it send queued data.
Session resume is helping a case when NAT change address and cause less
network traffic.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>