use state cache to optimise historical state lookup (#4228)

## Issue Addressed

#3873

## Proposed Changes

add a cache to optimise historical state lookup.

## Additional Info

N/A


Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/src/cli.rs

@@ -519,6 +519,13 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .help("Specifies how many blocks the database should cache in memory [default: 5]")
                 .takes_value(true)
         )
+        .arg(
+            Arg::with_name("historic-state-cache-size")
+                .long("historic-state-cache-size")
+                .value_name("SIZE")
+                .help("Specifies how many states from the freezer database should cache in memory [default: 1]")
+                .takes_value(true)
+        )
         /*
          * Execution Layer Integration
          */