Rename Historian to RecordStage

Historian was a legacy name. The new name reflects the new pipelined
architecture.

src/lib.rs

@@ -9,12 +9,12 @@ pub mod erasure;
 pub mod event;
 pub mod event_processor;
 pub mod hash;
-pub mod historian;
 pub mod ledger;
 pub mod logger;
 pub mod mint;
 pub mod packet;
 pub mod plan;
+pub mod record_stage;
 pub mod recorder;
 pub mod request;
 pub mod request_processor;