Feature/refactor client layer

[jeanparpaillon]

No description provided.

[johngiffin]

Looks like this branch causes DS tests to break, beyond just the new usage/options. Wasn't able to see immediately what the issue is. DS tests pass using the develop branch.

[johngiffin]

@jeanparpaillon There's still this example in the README which is outdated:

CoAP.Client.request(
  :con,
  method,
  url,
  request_payload,
  %{retries: retries, timeout: @wait_timeout, ack_timeout: timeout, tag: tag}
)

[jeanparpaillon]

Client part should be ready for full review.
Server part has not been updated. We may consider splitting into:

• coap - base, include UDP socket
• coapc - client
• coap_phoenix, coap_server ?
• In future, coap_adapter_* - like websocket, uart, tcp ?