Add shutdown argument to VBCSCompiler

[jaredpar]

This adds a -shutdown argument to VBCSCompiler that allows clients to gracefully shutdown the server. This is implemented by adding a new message to the protocol which does two actions:

• Begins a shut down of the server.
• Returns the process id of the server to the client who initiated the shutdown.

When reviewing the changes to ServerDispatcher it's important to understand the difference between the following operations:

• Shutting down: The server will accept no new build requests but will allow previously scheduled ones to complete without interruption. While these builds are finishing up the server will continue to respond to shutdown requests with the process id.
• Cancelling: This happens when the CancellationToken passed into the server is signaled. In this case the server will cancel all previously scheduled build requests and accept no new connections. In other words, cancel as fast as possible.

closes #7115

CC @dotnet/roslyn-compiler

[jaredpar]

retest please

[davkean]

@dotnet-bot retest prtest/lin/dbg/unit32 please
// Previous failure: http://dotnet-ci.cloudapp.net/job/roslyn_prtest_lin_dbg_unit32/2713/
// Retest reason: Build break

[agocke]

What's the motivation here?

[jaredpar]

@agocke there were an increasing number of scenarios where we wanted to shutdown a specific compiler server gracefully without waiting for the timeout to expire. Typically in order to get it to release references to the compiler DLLs. The scenarios we had hit:

• The Jenkins build scripts
• NuGet operations
• Roslyn VSIX install / uninstall

Killing all VBCSCompiler instances, while safe, is a bit hacky. Having a definitive shutdown seemed like a much more palatable option.

[AlekseyTs]

What does response Shutdown mean?

[AlekseyTs]

I think members of this enum deserve good comments.

[AlekseyTs]

LGTM

[VSadov]

LGTM