Convert Sync to Local

In order to write drop-in replacements for cache-dir for redis/s3/[the cloud](https://chrome.google.com/webstore/detail/cloud-to-butt-plus/apmlngnhgbnjpajelfkmabhkfapgnoai?hl=en), we can't actually support a *true* Sync set of methods. Instead they should be "Local" methods that will return approximately the right thing, while the default methods will return the right thing.

cc @jcorbin @markyen @Raynos 

(Please no bikeshedding on the name "Local", we spent half an hour on that already)