Implement receiving of request-line and headers
Rename rfkv_client to rfkv_handler

This better reflects its purpose; it's not a client.
Add rfkv_handler behaviour
Forward requests to request handler
Respect "Connection: keep-alive" header
Read request body

Only the default transfer encoding has been implemented, and reading the body
works only when the content-length header is present.
Handle "Content-Length: 0" correctly
Merge branch 'feature/http' into develop

* feature/http:
  Handle "Content-Length: 0" correctly
  Read request body
  Respect "Connection: keep-alive" header
  Forward requests to request handler
  Add rfkv_handler behaviour
  Rename rfkv_client to rfkv_handler
  Implement receiving of request-line and headers