add a batch_add feature to avoid sort for each virtual node addition

[dzhao]

in some of our setups we need to increase virtual nodes to 10k to get a better distribution of the data. This is to add a batch API to speedup the building process

[jeromefroe]

@dzhao the PR looks good to me. Can you just format your code and add a test before we merge?

[dzhao]

test added

[jeromefroe]

Thanks for the contribution @dzhao!