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/src/cli.rs

@@ -370,6 +370,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     address of this server (e.g., http://localhost:5054).")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("shuffling-cache-size")
+            .long("shuffling-cache-size")
+            .help("Some HTTP API requests can be optimised by caching the shufflings at each epoch. \
+            This flag allows the user to set the shuffling cache size in epochs. \
+            Shufflings are dependent on validator count and setting this value to a large number can consume a large amount of memory.")
+            .takes_value(true)
+        )
 
         /*
          * Monitoring metrics