Use Node workers

[avaer]

Workers (based on WebWorkers) are coming in Node 11: https://github.com/nodejs/node/labels/worker

Right now we hack around the lack of threads with child-process-thread but it's better to just use the node solution, which seems equivalent.

Note that this is not the same thing as our usage of vm-one, which is not about threading, but fast context isolation.

[avaer]

It's in node 10.5.0 under --experimental-worker: https://nodejs.org/en/blog/release/v10.5.0/