Optimize stakes cache and rewards at epoch boundaries (backport #20432) (#20472)

* Optimize stakes cache and rewards at epoch boundaries (backport #20432)

* fix conflicts

validator/src/main.rs

@@ -1904,6 +1904,14 @@ pub fn main() {
                 .help("Allow contacting private ip addresses")
                 .hidden(true),
         )
+        .arg(
+            Arg::with_name("disable_epoch_boundary_optimization")
+                .long("disable-epoch-boundary-optimization")
+                .takes_value(false)
+                .help("Disables epoch boundary optimization and overrides the \
+                optimize_epoch_boundary_updates feature switch if enabled.")
+                .hidden(true),
+        )
         .after_help("The default subcommand is run")
         .subcommand(
             SubCommand::with_name("exit")
@@ -2362,6 +2370,8 @@ pub fn main() {
         tpu_coalesce_ms,
         no_wait_for_vote_to_start_leader: matches.is_present("no_wait_for_vote_to_start_leader"),
         accounts_shrink_ratio,
+        disable_epoch_boundary_optimization: matches
+            .is_present("disable_epoch_boundary_optimization"),
         ..ValidatorConfig::default()
     };