Evaluate storing "buildplan" info in server node

There are several pluggable scheduler options (that are mostly unused). One of them, activated with `MSBUILDENABLEBUILDPLAN`, stores information about a build for use in subsequent scheduling decisions. After a single build with "buildplan" enabled, we know the project-to-project references and approximate project execution time, which we can use to schedule better by understanding what the critical path through the last build was.

This required writing a `.buildplan` file to disk before, because the scheduler lived in the entrypoint node that lived only for the execution of a single command-line build. But now, with MSBuild server, we have a place where we can store that information persistently in memory. We should try that.