Function create_writer 

pub fn create_writer(repo_path: &Path) -> (SaltCacheSender, SaltCacheSaver)

Create a paired sender/saver for collecting cache entries.

The sender is Sync and can be shared across rayon threads. The saver should be kept on the main thread and .save()d after parallel work completes. If .save() is not called, SaltCacheSaver::drop will persist any buffered entries as a safety net.