Install-UnitySetupInstance could be more asyncronous

[jwittner]

Right now it downloads them one at a time then installs all. It should download them all asynchronously, when the main installer for the platform is done it should install that, then it should install components as they finish downloading.

[jwittner]

Note: downloads are in parallel, but it still waits for all then installs, where it might install them as they become ready (after the core installer for the platform, of course).