Retransmit blobs from leader from window (#975)

- Some nodes don't have leader information while leader is broadcasting
  blobs to those nodes. Such blobs are not retransmitted. This change
  rertansmits the blobs once the leader's identity is know.

src/erasure.rs

@@ -713,7 +713,8 @@ mod test {
         let mut window = vec![
             WindowSlot {
                 data: None,
-                coding: None
+                coding: None,
+                leader_unknown: false,
             };
             WINDOW_SIZE
         ];