* Cleanup code for distributed training.
- Merge `GetNcclResult` into nccl stub.
- Split up utilities from the main dask module.
- Let Channel return `Result` to accommodate nccl channel.
- Remove old `use_label_encoder` parameter.
* [coll] Implement a new tracker and a communicator.
The new tracker and communicators communicate through the use of JSON documents. Along
with which, communicators are aware of each other.
