[MSBuild Server] Define and handle build request message

[rokonec]

Using existing infrastructure code, define and handle message (INodePacket) for MSBuild Server request.

Work breakdown:

• define message format: (header(type, len), string(workingDirectory), string(command line)
• handle 'MSBuild Server request' message by calling MSBuild entry point
• use named mutex to guarantee build exclusivity (no concurrent builds are allowed)
• if build is already running reply with message MSBuildServerBusy - that will be handled by caller by fallbacking to standard non-server entry point (running new MSBuild process, loading MSBuild.dll into CLI and start it, ...)
• after build is finished reply with message MSBuildServerDone containing same exit code as would MSBuild process returned
• introduce ETL events to trace and measure perf of above

[rokonec]

Implemented and parked into https://github.com/dotnet/msbuild/tree/feature/msbuild-server-v2--parked

Work left:

• unit tests
• refactoring and cleaning