Add scalable gossip library (#1546)

* Cluster Replicated Data Store

Separate the data storage and merge strategy from the network IO boundary.
Implement an eager push overlay for transporting recent messages.

Simulation shows fast convergence with 20k nodes.

src/replicator.rs

@@ -89,9 +89,9 @@ impl Replicator {
 
         let leader_info = network_addr.map(|i| NodeInfo::new_entry_point(&i));
         let leader_pubkey;
-        if let Some(leader_info) = leader_info.as_ref() {
+        if let Some(leader_info) = leader_info {
             leader_pubkey = leader_info.id;
-            cluster_info.write().unwrap().insert(leader_info);
+            cluster_info.write().unwrap().insert_info(leader_info);
         } else {
             panic!("No leader info!");
         }