Implement suggestions from the new Webpack caching guide

[edmorley]

The webpack caching docs have just been updated in webpack/webpack.js.org#1436 to include some of the best practices from the blog post that inspired the current neutrino caching implementation.

There are probably going to be some further tweaks to the guide over the coming days/weeks, but once it stabilises, we should see if there are changes we should make to Neutrino to match it.

[edmorley]

With webpack 4 (#737) there are more sensible defaults out of the box, and if those defaults are not enough, IMO we should push to fix them upstream instead.