[FUTURE] Write a comparison benchmark using `worker_threads` module

Nobody has claimed this yet.

v10.5.0 has a new `worker_threads` module that allows pure JS functions to run on a separate thread. The node-cpp-skel demonstrates doing this type of thing using C++, but the `worker_threads` module allows for it in pure JS. (https://nodejs.org/en/blog/release/v10.5.0/)

It would be useful to try to write an 🍏 to 🍏 (or as close as we can get) comparison benchmark of doing the same work in C++ threads vs JS threads using `worker_threads`. We would try to answer questions like:

- 🍒 Is performance of passing data (#69, #67) similar in with `worker_threads` vs using libuv directly in C++?
- 🍊 When combined (overhead of thread queuing, passing data to the thread, doing compute, getting data back) which approach is faster?

We could get at 🍒 by writing a benchmark that passes the same, large data, and does no compute in the thread. Instead that thread could just sleep for N seconds.

We could get at 🍊 by passing the same data and doing the same (as close as possible) compute, perhaps a Fibonacci (or some kind of classical compute intensive workload).

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Depending on the results we will want to update https://github.com/mapbox/cpp/blob/master/node-cpp.md#why-create-a-node-addon which says:

> Why create a node addon?
> To port a C++ project to Node to expose a new interface for the tool (like Mapnik & Node Mapnik)
> Improve performance at scale where Node becomes the bottleneck (i.e. concurrency)

If JS is just as fast as C++, then we'll want to update that doc to mention `worker_threads` as a more ideal solution than writing a node addon to access the thread pool. If `JS/worker_threads` is slower than C++ then we'll probably want to add details to that doc about what types of workloads or performance situations we'd want to use C++ and which we'd want to consider `JS/worker_threads`

/cc @mapbox/core-tech

Start by reviewing the node-cpp-skel examples and the node-cpp.md section linked in the issue, along with the referenced worker_threads release information. Define comparable C++ and JavaScript worker benchmarks for data passing and compute, then use their results to determine whether node-cpp.md needs updated guidance.

Written by the indexing model from the issue text.