Move options related types that are linked into CodeStyle layer to a different namespace to avoid type name conflicts

Extracts out the changes to core Workspace changes from https://github.com/dotnet/roslyn/pull/41363.

... into a static helper class and share it at CSharpFeatures level.
This is the first step
towards getting the codegen features (introduce local, generate
variable) to be able
to use the new use var options correctly.

Added a test to verify the shutdown operation is passive.  If any
existing compilations are in progress they will be allowed to complete
after the shutdown is received.

This required a bit of code refactoring.  It needed to be easier to
intercept compilation requests and block on them until a given set of
events had completed.

closes #4301

Use a base type to simplify and unify the hosting of CompilerServerHost.

It's no longer needed on the ICompilerServerHost implementation.  It is
just threaded through the compiler server creation like the other
directory parameters.

This will be the replacement for CompilerRequestHandler in the portable
layer

     Background:

     As discussed in the dev team all hands all code under the GitHub dotnet foundation is using a single process for contribution, API review, infrastructure and coding style. The idea is to present a unified view to our customer and give them a single story for contributing to any project under the dotnet foundation.

     https://github.com/dotnet/corefx/wiki/Contributing#c-coding-style

     The coding style transition is automated using a Roslyn based rewrite tool:

     https://github.com/dotnet/codeformatter

     This will be applied in stages across our developer tree. Right now the focus is on the Open directory as this is what is being presented on github. Code owners will be contacted before the transition happens.

     Note: this is a soft style requirement. There are no build errors that come from this change.
 (changeset 1408227)