One-shot execution mode for build servers

[baronfel]

Is your feature request related to a problem? Please describe.

We should provide a flag for the various build-related commands that instructs any build servers invoked during the build to shut down at the end of build execution. This flag would need to

• signal to the msbuild execution engine to clean up owned nodes (or spawn the nodes in such a way that they clean themselves up after each build finishes)
• flow the user signal to Roslyn/Razor in some way that allows those areas to clean their own processes up. Note that this means the signal can't really be passed to the actual Csc task or equivalent Razor task - some kind of AfterBuild target might be needed to do the cleanup.

This is requested by the VMR (cc @ViktorHofer ) to make build isolation easier and reduce locking.