net: lwm2m: move to using the new multi-packet CoAP API

This patch moves from the ZoAP API in subsys/net/lib/zoap to
the CoAP API in subsys/net/lib/coap which handles multiple
fragments for sending / receiving data.

NOTE: This patch moves the LwM2M library over to the CoAP APIs
but there will be a follow-up patch which re-writes the content
formatter reader / writers to use net_pkt APIs for parsing
across multiple net buffers. The current implementation assumes
all of the data will land in 1 buffer.

Samples using the library still need a fairly large NET_BUF_DATA_SIZE
setting. (Example: CONFIG_NET_BUF_DATA_SIZE=384)

Signed-off-by: Michael Scott <michael.scott@linaro.org>