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/serde_utils.rs

@@ -0,0 +1,62 @@
+use std::fmt;
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+
+struct U64Visitor;
+impl<'a> serde::de::Visitor<'a> for U64Visitor {
+    type Value = u64;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("Expecting u64")
+    }
+    fn visit_u64<E>(self, data: u64) -> std::result::Result<u64, E>
+    where
+        E: serde::de::Error,
+    {
+        Ok(data)
+    }
+}
+
+pub fn deserialize_atomicusize<'de, D>(d: D) -> Result<AtomicUsize, D::Error>
+where
+    D: serde::de::Deserializer<'de>,
+{
+    let value = d.deserialize_u64(U64Visitor)?;
+    Ok(AtomicUsize::new(value as usize))
+}
+
+pub fn serialize_atomicusize<S>(x: &AtomicUsize, s: S) -> Result<S::Ok, S::Error>
+where
+    S: serde::Serializer,
+{
+    s.serialize_u64(x.load(Ordering::SeqCst) as u64)
+}
+
+struct BoolVisitor;
+impl<'a> serde::de::Visitor<'a> for BoolVisitor {
+    type Value = bool;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("Expecting bool")
+    }
+    fn visit_bool<E>(self, data: bool) -> std::result::Result<bool, E>
+    where
+        E: serde::de::Error,
+    {
+        Ok(data)
+    }
+}
+
+pub fn deserialize_atomicbool<'de, D>(d: D) -> Result<AtomicBool, D::Error>
+where
+    D: serde::de::Deserializer<'de>,
+{
+    let value = d.deserialize_bool(BoolVisitor)?;
+    Ok(AtomicBool::new(value))
+}
+
+pub fn serialize_atomicbool<S>(x: &AtomicBool, s: S) -> Result<S::Ok, S::Error>
+where
+    S: serde::Serializer,
+{
+    s.serialize_bool(x.load(Ordering::SeqCst))
+}