Adjusting output severities

http response status and timing should have same treatment as request
report severity enum should not divide output between stdout and stderr
Remove any unprocessed projects from remote

#794
Initial plumbing of IAssemblyLoadContext and family
- Removed IAssemblyLoaderEngine and replace it with IAssemblyLoadContext
- Added Microsoft.Framework.Runtime.Loader which has implementations of
IAssemblyLoadContext and IAssemblyLoadContextAccessor for net45 and aspnetcore50
- Use current LoadContext to LoadFile and LoadStream in different loaders
Added IAssemblyLoadContextFactory abstraction
- This interface creates a IAssemblyLoadContext with relevant loaders
- Updated the NuGetAssemblyLoader and ProjectAssemblyLoader to allow passing
in an IAssemblyLoadContext.
- Updated the RoslynCompiler to create a child context when loading the
preprocess assembly.
- Made IAssemblyLoadContext disposable
- Fixed issues with LoadContext and infinite recursion
- Added IAssemblyLoaderEngine for compatibility