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

runtime/src/stakes.rs

@@ -1,11 +1,11 @@
 //! Stakes serve as a cache of stake and vote accounts to derive
 //! node stakes
-use hashbrown::HashMap;
 use solana_sdk::account::Account;
 use solana_sdk::pubkey::Pubkey;
 use solana_stake_api::stake_state::StakeState;
+use std::collections::HashMap;
 
-#[derive(Default, Clone)]
+#[derive(Default, Clone, PartialEq, Debug, Deserialize, Serialize)]
 pub struct Stakes {
     /// vote accounts
     vote_accounts: HashMap<Pubkey, (u64, Account)>,