Improve freezer DB efficiency with periodic restore points (#649)

* Draft of checkpoint freezer DB

* Fix bugs

* Adjust root iterators for checkpoint database

* Fix freezer state lookups with no slot hint

* Fix split comment

* Use "restore point" to refer to frozen states

* Resolve some FIXMEs

* Configurable slots per restore point

* Document new freezer DB functions

* Fix up StoreConfig

* Fix new test for merge

* Document SPRP default CLI flag, clarify tests

beacon_node/store/src/config.rs

@@ -0,0 +1,32 @@
+use serde_derive::{Deserialize, Serialize};
+use std::path::PathBuf;
+use types::{EthSpec, MinimalEthSpec};
+
+/// Default directory name for the freezer database under the top-level data dir.
+const DEFAULT_FREEZER_DB_DIR: &str = "freezer_db";
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct StoreConfig {
+    /// Name of the directory inside the data directory where the main "hot" DB is located.
+    pub db_name: String,
+    /// Path where the freezer database will be located.
+    pub freezer_db_path: Option<PathBuf>,
+    /// Number of slots to wait between storing restore points in the freezer database.
+    pub slots_per_restore_point: u64,
+}
+
+impl Default for StoreConfig {
+    fn default() -> Self {
+        Self {
+            db_name: "chain_db".to_string(),
+            freezer_db_path: None,
+            slots_per_restore_point: MinimalEthSpec::slots_per_historical_root() as u64,
+        }
+    }
+}
+
+impl StoreConfig {
+    pub fn default_freezer_db_dir(&self) -> &'static str {
+        DEFAULT_FREEZER_DB_DIR
+    }
+}