feat: improve prepareNextEpoch

[twoeths]

Motivation

Reduce prepareNextEpoch time for lodestar

Description

• use hashtree for batch hash implemented in ssz

  • copy the implementation of ListValidatorTreeViewDU from the ssz branch, the main thing is to compute validators' roots in batch during commit() phase
  • batch hash validators, execute HashComputation[] in batch

• avoid memory allocation:

  • no tree creation when we modify/add validator

• reuse balances tree ==> although this helps save ~250ms to 300ms on epoch transition time, this may introduce unpredictable behavior so I decided not to go with it

[twoeths]

this is 6h rate interval on lg1k

• PrepareNextEpoch is around 850ms (1.5s on beta)

• Block transition hash_tree_root is 27ms (vs 70ms on beta), epoch transition hash_tree_root is 130ms (vs 533ms on beta)