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/locktower.rs

@@ -1,11 +1,11 @@
 use crate::bank_forks::BankForks;
 use crate::staking_utils;
-use hashbrown::{HashMap, HashSet};
 use solana_metrics::influxdb;
 use solana_runtime::bank::Bank;
 use solana_sdk::account::Account;
 use solana_sdk::pubkey::Pubkey;
 use solana_vote_api::vote_state::{Lockout, Vote, VoteState, MAX_LOCKOUT_HISTORY};
+use std::collections::{HashMap, HashSet};
 use std::sync::Arc;
 
 pub const VOTE_THRESHOLD_DEPTH: usize = 8;