general restructuring, reimplement CoapNode into EddieEndpoint, extract resource directory code, fixes and improvements

This large MR is a general restructuring of the code, but does not add any new feature. These changes, other than improving the code, also allows to share some code with the zephyr implementation.

• Remove CoapNode and reimplement it into EddieEndpoint sharing the same header file used in zephyr
• Refactor CoapServer.cpp extracting the resource directory code into its own class: improves readability of CoapServer.cpp file and also allows to run the resource directory as a standalone process.
• replace printf and coap_log with LOG_ERR, LOG_INFO, LOG_DBG: this allows to set wanted log verbosity, or in the future log to a different location instead of stdout
• fix segmentation fault problem
• move send_message and receive_message methods into CoapClient
• adapt virtualization to use EddieEndpoint instead of CoapNode

for more details see the git history

Signed-off-by: Francesco Pham francesco.pham@huawei.com