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/locktower.rs

@@ -1,13 +1,12 @@
 use crate::bank_forks::BankForks;
 use crate::staking_utils;
-use hashbrown::{HashMap, HashSet};
 use solana_metrics::datapoint_info;
 use solana_runtime::bank::Bank;
 use solana_sdk::account::Account;
 use solana_sdk::hash::Hash;
 use solana_sdk::pubkey::Pubkey;
 use solana_vote_api::vote_state::{Lockout, Vote, VoteState, MAX_LOCKOUT_HISTORY};
-use std::collections::VecDeque;
+use std::collections::{HashMap, HashSet, VecDeque};
 use std::sync::Arc;
 
 pub const VOTE_THRESHOLD_DEPTH: usize = 8;