Customisable shuffling cache size (#4081)

This PR enables the user to adjust the shuffling cache size.

This is useful for some HTTP API requests which require re-computing old shufflings. This PR currently optimizes the
beacon/states/{state_id}/committees HTTP API by first checking the cache before re-building shuffling.

If the shuffling is set to a non-default value, then the HTTP API request will also fill the cache when as it constructs new shufflings.

If the CLI flag is not present or the value is set to the default of 16 the default behaviour is observed.


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

beacon_node/beacon_chain/src/chain_config.rs

@@ -67,6 +67,8 @@ pub struct ChainConfig {
     pub prepare_payload_lookahead: Duration,
     /// Use EL-free optimistic sync for the finalized part of the chain.
     pub optimistic_finalized_sync: bool,
+    /// The size of the shuffling cache,
+    pub shuffling_cache_size: usize,
     /// Whether to send payload attributes every slot, regardless of connected proposers.
     ///
     /// This is useful for block builders and testing.
@@ -97,6 +99,7 @@ impl Default for ChainConfig {
             prepare_payload_lookahead: Duration::from_secs(4),
             // This value isn't actually read except in tests.
             optimistic_finalized_sync: true,
+            shuffling_cache_size: crate::shuffling_cache::DEFAULT_CACHE_SIZE,
             always_prepare_payload: false,
         }
     }