websocket data parser upates

- Renamed hook names:
    on_msg_begin    -> on_msg_start
    on_msg_payload  -> on_msg_data
    on_msg_complete -> on_msg_fini

- Added evhtp_ws_parser_(set|get)_userdata() functions.

- Once we hit the ws_s_payload case statement, and the frame pcode is CLOSE, we
  now properly deal with decoding the close status code and if available, the
  message afterwards.

- the on_msg_fini callback now is not called until the websocket frame's fin bit
  is set.

- removed the packed attribute from the frame_hdr structure since it's already
  aligned properly.