Use pnet_datalink instead of all of pnet

pnet_transport takes a long time to build. It's been especially
painful from within a docker container for reasons I don't care
to understand. pnet_datalink is the only part of pnet we're using
so booting the rest.

src/bin/client-demo.rs

@@ -1,7 +1,6 @@
 extern crate atty;
 extern crate env_logger;
 extern crate getopts;
-extern crate pnet;
 extern crate rayon;
 extern crate serde_json;
 extern crate solana;