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/benches/status_cache.rs

@@ -0,0 +1,33 @@
+#![feature(test)]
+
+extern crate test;
+
+use bincode::serialize;
+use solana_runtime::status_cache::*;
+use solana_sdk::hash::{hash, Hash};
+use solana_sdk::signature::Signature;
+use test::Bencher;
+
+type BankStatusCache = StatusCache<()>;
+
+#[bench]
+fn test_statuscache_serialize(bencher: &mut Bencher) {
+    let mut status_cache = BankStatusCache::default();
+    status_cache.add_root(0);
+    status_cache.clear_signatures();
+    for hash_index in 0..100 {
+        let blockhash = Hash::new(&vec![hash_index; std::mem::size_of::<Hash>()]);
+        let mut id = blockhash;
+        for _ in 0..100 {
+            id = hash(id.as_ref());
+            let mut sigbytes = Vec::from(id.as_ref());
+            id = hash(id.as_ref());
+            sigbytes.extend(id.as_ref());
+            let sig = Signature::new(&sigbytes);
+            status_cache.insert(&blockhash, &sig, 0, ());
+        }
+    }
+    bencher.iter(|| {
+        let _ = serialize(&status_cache).unwrap();
+    });
+}