Create bank snapshots (#4244)

* Revert "Revert "Create bank snapshots (#3671)" (#4243)"

This reverts commit 81fa69d347.

* keep saved and unsaved copies of status cache

* fix format check

* bench for status cache serialize

* misc cleanup

* remove appendvec storage on purge

* fix accounts restore

* cleanup

* Pass snapshot path as args

* Fix clippy

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;