strip out all ICMP network code and begin gossip refactor for attestations (#256)

* strip out all ICMP code and begin gossip refactor

* validate incoming statements

* message_allowed logic

* compiles

* do reporting and neighbor packet validation

* tests compile

* propagate gossip messages

* test message_allowed

* some more tests

* address grumbles

polkadot/network/src/lib.rs

@@ -31,10 +31,8 @@ extern crate polkadot_primitives;
 extern crate arrayvec;
 extern crate parking_lot;
 extern crate tokio;
-extern crate slice_group_by;
 extern crate exit_future;
 
-#[macro_use]
 extern crate futures;
 #[macro_use]
 extern crate log;