Create a LocalCache class that can directly read a mounted S3 bucket as its cache_dir

[danielsf]

Some time ago, Nick made an addition for LocalStaticCache which lets an AWS Sagemaker user, (e.g.) mount the release bucket directly as a cache_dir when mounted as a drive via s3fs. Unfortunately, this does not work if for instance, we establish a shared isilon copy of the cache, which we want to be single-user managed and multi-user read. The immediate problem is that the directory structure of the downloaded cache does not have a manifests folder.