Perform R package discovery in a separate process

Add a client and server wrapper around RInterop for package discovery to
prevent VisualStudio from crashing if there are concurrency or other
errors related to accessing R through RDotNet.

Cleanup obsolete commented code.

Hardening stability and recovery of RInteropServer

The RInteropServer was still prone to crashes and was failing to recover
after a crash. These fixes prevent multiple threads from calling the
server at once and avoid an unnecessary R eval that was leading to
crashes under heavy concurrent load.

Add exe files to lookup assembly lookup

The RProvider.Runtime assembly is now an exe, so exe files need to be
included in the assembly resolution for libraries.