Explore parallelising the compilation

[satazor]

See https://github.com/trivago/parallel-webpack

[Mouvedia]

Iv got one problem with this: I need the client manifest before being able to build the server bundle. If it's an option and its default is false, I am fine with it.

cf https://ssr.vuejs.org/en/build-config.html#generating-clientmanifest

keywords: sequential, parallel