feat: ListUintNum64Type create ViewDU from existing tree

[twoeths]

Motivation

• In lodestar, at every epoch transition we create a new balances tree using ListUintNum64Type.toViewDU() api. This PR add an optional unusedViewDU to that api which helps reuse balances tree and avoid memory allocation in lodestar

Description

• This PR is technically not related to batch hash work, however using executeHashComputation() is a great way to rehash the whole tree
• The implementation is very similar to pushing new values to an existing ViewDU, then commit()
• Unit test and perf test are added. Although perf test does not show a great improvement, memory would be saved a lot.