Roslyn build server shutdown should use pid files for server discovery.

[peterhuene]

This is a tracking issue for future feature work.

Steps to reproduce

1. dotnet build-server shutdown

Expected behavior

No messages are printed out about the VB/C# compiler server shutting down unless it was actually running.

Actual behavior

It always prints a message about the VB/C# compiler server shutting down. This is because the implementation does not dynamically discover the running servers (like the Razor implementation does) using pid files written out by the server. It always attempts to shutdown the server based off the current SDK, rather than shutting down all running VB/C# compiler server instances like users expect.

Environment data

.NET Core SDK (reflecting any global.json):
 Version:   2.1.300-preview3-008647
 Commit:    00b6dfa681

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  10.13
 OS Platform: Darwin
 RID:         osx-x64
 Base Path:   /Users/peterhuene/src/cli/bin/2/osx-x64/dotnet/sdk/2.1.300-preview3-008647/

Host (useful for support):
  Version: 2.1.0-preview3-26411-06
  Commit:  8faa8fcfcf

.NET Core SDKs installed:
  2.1.300-preview3-008647 [/Users/peterhuene/src/cli/bin/2/osx-x64/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.0-preview2-30475 [/Users/peterhuene/src/cli/bin/2/osx-x64/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.0-preview2-30475 [/Users/peterhuene/src/cli/bin/2/osx-x64/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.0-preview3-26411-06 [/Users/peterhuene/src/cli/bin/2/osx-x64/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download