ViewDU container rebinds node with read-only operations

[twoeths]

Describe the bug

Consider this unit test here in #378

  it("should not recompute hashTreeRoot() when no fields is changed", () => {
    const root = state.hashTreeRoot();
    // this causes viewsChanged inside BeaconState container
    state.validators.length;
    state.balances.length;
    // but we should not recompute root, should get from cache instead
    const root2 = state.hashTreeRoot();
    expect(root2).to.equal(root, "should not recompute hashTreeRoot() when no fields are changed");
  });

the BeaconState container tracked validators and length as "viewsChanged" and it rebinds the underlying tree to compute new root with same nodes, which results in the same value but different tree/nodes, and we need to rehash the different nodes

note that #378 implemented a cache that returns the same root for multiple ViewDU.hashTreeRoot() calls

Expected behavior

• according to the design of ssz v2, BeaconState container should still track validators and balances as "viewsChanged" so that when we modify them they will escalate the change to BeaconState. However when we commit those viewDUs, if we see no change we should not rebind the nodes, and it should return the same cached root

Steps to Reproduce

• Run above unit test on #378

[philknows]

@twoeths Can this be closed via #380?

[twoeths]

@twoeths Can this be closed via #380?

@philknows not yet since I merged it to the te/batch_hash_tree_root branch. Will close it once we merge to master