Use BTreeMap for state.validators pending updates (#9017)

Closes:

- https://github.com/sigp/lighthouse/issues/9003


  Milhouse `List`s use a map in front of the binary tree to cache updates. Ever since we adopted Milhouse, we've been using `VecMap`, which is essentially `Vec<Option<T>>`. Turns out, when you've got 2M indices and only 2 non-`None` entries (changes), this is inefficient.

Milhouse is generic in the choice of map (`U: UpdateMap`) and has always supported `BTreeMap`, so this PR switches us over to `BTreeMap`. In previous benchmarks (years ago) it had been slower than `VecMap`, but now it is vastly superior.


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

consensus/types/src/state/beacon_state.rs

@@ -14,6 +14,7 @@ use serde::{Deserialize, Deserializer, Serialize};
 use ssz::{Decode, DecodeError, Encode, ssz_encode};
 use ssz_derive::{Decode, Encode};
 use ssz_types::{BitVector, FixedVector};
+use std::collections::BTreeMap;
 use superstruct::superstruct;
 use swap_or_not_shuffle::compute_shuffled_index;
 use test_random_derive::TestRandom;
@@ -58,7 +59,8 @@ pub const CACHED_EPOCHS: usize = 3;
 const MAX_RANDOM_BYTE: u64 = (1 << 8) - 1;
 const MAX_RANDOM_VALUE: u64 = (1 << 16) - 1;
 
-pub type Validators<E> = List<Validator, <E as EthSpec>::ValidatorRegistryLimit>;
+pub type Validators<E> =
+    List<Validator, <E as EthSpec>::ValidatorRegistryLimit, BTreeMap<usize, Validator>>;
 pub type Balances<E> = List<u64, <E as EthSpec>::ValidatorRegistryLimit>;
 
 #[derive(Debug, PartialEq, Clone)]
@@ -453,7 +455,7 @@ where
     // Registry
     #[compare_fields(as_iter)]
     #[test_random(default)]
-    pub validators: List<Validator, E::ValidatorRegistryLimit>,
+    pub validators: Validators<E>,
     #[serde(with = "ssz_types::serde_utils::quoted_u64_var_list")]
     #[compare_fields(as_iter)]
     #[test_random(default)]

consensus/types/src/state/mod.rs

@@ -17,7 +17,7 @@ pub use balance::Balance;
 pub use beacon_state::{
     BeaconState, BeaconStateAltair, BeaconStateBase, BeaconStateBellatrix, BeaconStateCapella,
     BeaconStateDeneb, BeaconStateElectra, BeaconStateError, BeaconStateFulu, BeaconStateGloas,
-    BeaconStateHash, BeaconStateRef, CACHED_EPOCHS,
+    BeaconStateHash, BeaconStateRef, CACHED_EPOCHS, Validators,
 };
 pub use committee_cache::{
     CommitteeCache, compute_committee_index_in_epoch, compute_committee_range_in_epoch,