thread-loader's 'poolTimeout' should be overridden when watching

Created by: edmorley

Hi!

Currently create-react-app uses thread-loader with default options in both the development and production configs: https://github.com/facebook/create-react-app/blob/bf3d73c2c6fd78d4eea23f5356fb26f3232fabee/packages/react-scripts/config/webpack.config.dev.js#L211-L213 https://github.com/facebook/create-react-app/blob/bf3d73c2c6fd78d4eea23f5356fb26f3232fabee/packages/react-scripts/config/webpack.config.prod.js#L237-L239

The production usage is fine, however as mentioned here, when watching is enabled (ie the development config with webpack-dev-server) the poolTimeout option should be changed from its default of 500 ms to Infinity to avoid paying the overhead of worker startup on every rebuild.

See: https://github.com/webpack-contrib/thread-loader#examples