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/tests/cluster_info.rs

@@ -1,9 +1,9 @@
-use hashbrown::{HashMap, HashSet};
 use rayon::iter::{IntoParallelIterator, ParallelIterator};
 use rayon::prelude::*;
 use solana::cluster_info::{compute_retransmit_peers, ClusterInfo};
 use solana::contact_info::ContactInfo;
 use solana_sdk::pubkey::Pubkey;
+use std::collections::{HashMap, HashSet};
 use std::sync::mpsc::channel;
 use std::sync::mpsc::TryRecvError;
 use std::sync::mpsc::{Receiver, Sender};