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