HTTPServer: Update the Test server to be concurrent and improve reading requests

- _TCPSocket is now only used for 1 socket, either a listener or client
  connection.

- _HTTPServer now listens for a connection and returns a new _HTTPServer with
  the client connection.

- Client _HTTPServer sub-servers are run in a Dispatch async() block to
  provide conncurrency for incoming connections.

- When reading the request from the socket, _SocketDataReader() is now used to
  read, as `Data` not `String`, and look for appropiate HTTP separators (\r\n).

- When reading chunked requests check for the length of each chunk and look for
  the terminating 0 byte chunk.

Author: spevans