During the CoAP API change, slight changes were made the ref / unref
packet pending process. Let's re-align with the coap-client sample
in how we apply the packet refs in retransmit_request() and also
replace the lwm2m_send_message() call with a direct call of
net_app_send_pkt(). This avoids a second processing of the pending
packets and keeps the ref/unref flow cleaner.
Signed-off-by: Michael Scott <michael@opensourcefoundries.com>