Prevent writing to state cache when migrating the database (#7067)

* add an update_cache flag to get_state to have more granular control over when we write to the cache

* State cache tweaks

- add state-cache-headroom flag to control pruning
- prune old epoch boundary states ahead of mid-epoch states
- never prune head block's state
- avoid caching ancestor states unless they are on an epoch boundary

---------

Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/src/cli.rs

@@ -792,6 +792,15 @@ pub fn cli_app() -> Command {
                 .action(ArgAction::Set)
                 .display_order(0)
         )
+        .arg(
+            Arg::new("state-cache-headroom")
+                .long("state-cache-headroom")
+                .value_name("N")
+                .help("Minimum number of states to cull from the state cache when it gets full")
+                .default_value("1")
+                .action(ArgAction::Set)
+                .display_order(0)
+        )
         .arg(
             Arg::new("block-cache-size")
                 .long("block-cache-size")