The driver plugin stub client must call `grpcutils.HandleGrpcErr` to handle plugin
shutdown similar to other functions.  This ensures that TaskStats returns
`ErrPluginShutdown` when plugin shutdown.

Previously, if a channel is closed, we retry the Stats call.  But, if that call
fails, we go in a backoff loop without calling Stats ever again.

Here, we use a utility function for calling driverHandle.Stats call that retries
as one expects.

I aimed to preserve the logging formats but made small improvements as I saw fit.