Move ledger write to its own stage (#1577)

* Move ledger write to its own stage

- Also, rename write_stage to leader_vote_stage, as write functionality
  is moved to a different stage

* Address review comments

* Fix leader rotation test failure

* address review comments

src/lib.rs

@@ -34,7 +34,9 @@ pub mod fetch_stage;
 pub mod fullnode;
 pub mod hash;
 pub mod leader_scheduler;
+pub mod leader_vote_stage;
 pub mod ledger;
+pub mod ledger_write_stage;
 pub mod loader_transaction;
 pub mod logger;
 pub mod metrics;
@@ -80,7 +82,6 @@ pub mod vote_stage;
 pub mod wallet;
 pub mod window;
 pub mod window_service;
-pub mod write_stage;
 extern crate bincode;
 extern crate bs58;
 extern crate byteorder;