Create bank snapshots (#3671)

* Be able to create bank snapshots

* fix clippy

* load snapshot on start

* regenerate account index from the storage

* Remove rc feature dependency

* cleanup

* save snapshot for slot 0

core/src/blocktree.rs

@@ -11,7 +11,7 @@ use solana_kvstore as kvstore;
 
 use bincode::deserialize;
 
-use hashbrown::HashMap;
+use std::collections::HashMap;
 
 #[cfg(not(feature = "kvstore"))]
 use rocksdb;