This is an experiment to use
[xstate](http://davidkpiano.github.io/xstate/docs/#/) to setup state
machines to better handle complex state changes as we sometimes have.

Ideally this happens in core and then gatsby-source-filesystem
just has a simple queue and emits a new file node every time
the system returns to idle.

In a future refactor we'll do that plus refactor other parts of core
that should be handled in a state machine e.g. pages-query-runner.js

This PR also reinforced the need for us to implement
[tracing](https://github.com/jaegertracing/jaeger) in core / plugins
as that'd make it far far easier to understand what's happening and
when.