more restructuring, ipv6 fixes, parse program arguments, refactor CoapClient, add tests

• parse ip, port parameters using argparse
• move client coap_io_process thread into CoapClient
• refactor CoapClient to remove duplicate code
• fix ipv6 problems by properly formatting url
• add send_message_and_wait_response to avoid boilerplate when sending simple requests
• linux/CoapClient: stop waiting as soon as the response is received
• tests: add fixture to simulate two nodes communicating
• add tests
• fix GVariant format string not properly formatted
• other fixes and improvements

For more details see the git history

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