Make the block cache optional (#8066)

Address contention on the store's `block_cache` by allowing it to be disabled when `--block-cache-size 0` is provided, and also making this the default.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/src/cli.rs

@@ -779,7 +779,7 @@ pub fn cli_app() -> Command {
                 .long("block-cache-size")
                 .value_name("SIZE")
                 .help("Specifies how many blocks the database should cache in memory")
-                .default_value("5")
+                .default_value("0")
                 .action(ArgAction::Set)
                 .display_order(0)
         )