Download unknown but announced forks (#1604)

* test reproducing fork sync issues

* update to new announce_block API

* Download unknown forks

* Reverted download_stale

* Avoid cloning the hash

* Typo

substrate/core/finality-grandpa/src/communication.rs

@@ -17,6 +17,7 @@
 //! Incoming message streams that verify signatures, and outgoing message streams
 //! that sign or re-shape.
 
+use std::collections::HashMap;
 use grandpa::VoterSet;
 use futures::prelude::*;
 use futures::sync::mpsc;