Rocks db (#1792)

* Add rocksdb crate

* Implement new ledger module based on RocksDb

src/lib.rs

@@ -35,6 +35,7 @@ pub mod contact_info;
 pub mod budget_program;
 pub mod cluster_info;
 pub mod compute_leader_finality_service;
+pub mod db_ledger;
 pub mod drone;
 pub mod entry;
 #[cfg(feature = "erasure")]
@@ -114,6 +115,7 @@ extern crate pnet_datalink;
 extern crate rayon;
 extern crate reqwest;
 extern crate ring;
+extern crate rocksdb;
 extern crate serde;
 #[macro_use]
 extern crate serde_derive;