Making the gpr_tmpfile's win32 version a bit more Windows-y.

V0 implementation of census_get_active_ops().

Use environment variable to set root certs in c++ interop test

Adding /tmp to prefix when creating posix temporary files.

- Addresses #416.

Also adding windows helpers to convert to and from TCHAR strings.

Improved C# grpc server implementation to be able to register call handlers

Move pollset_kick wfd creation/destruction out of freelist lock

This was an observed source of contention at higher thread counts where
we could overrun the freelist cap.

Adding support for loading the SSL roots from an environment variable.

Added usage info to csharp/README.md