Replicator rework

* Move more of the replicator logic into the replicator class
* Add support for the RPC interface to query the storage last_id value
  that the replicator would sign and use to pick a block.
* Fix replicator connecting to gossip and change test to exercise that
  scenario.

src/rpc_request.rs

@@ -53,6 +53,7 @@ pub enum RpcRequest {
     RegisterNode,
     SignVote,
     DeregisterNode,
+    GetStorageMiningLastId,
 }
 
 impl RpcRequest {
@@ -95,6 +96,7 @@ impl RpcRequest {
             RpcRequest::RegisterNode => "registerNode",
             RpcRequest::SignVote => "signVote",
             RpcRequest::DeregisterNode => "deregisterNode",
+            RpcRequest::GetStorageMiningLastId => "getStorageMiningLastId",
         };
         let mut request = json!({
            "jsonrpc": jsonrpc,