A simpler caching file-system

[martindurant]

The caching file-system downloads from other file systems to a local store, and uses MMap to only fetch the chunks that are accessed, to a sparse file. An alternative mode of the same class would simply download whole files on first access, potentially with on-the-fly decompression, and then provide the local copy. This could then replace most/all of the functionality in Intake's cache layer (although who takes responsibility for metadata for checkpointing remains to be decided).