Cache pruning should be by LRU

[mnightingale]

Currently the oldest files (by modified time) are purged from the cache.

It'd be better if the least recently used ones were purged first.

Complicated a bit by the removal of permanent and move to CacheStrategy.
Mutable relies on the modified time of the local file but we can't just touch the file on every request for possible performance reasons (IO) and it would mean you couldn't switch from doing immutable to mutable requests because you might have set the modified time on an out of date file.

Possible solution is a separate key/value cache keeping track of the access time for each uri.
It would also be useful for tracking response headers to improve the CacheStrategy.mutable (use the actual modified time and etag)

[dandresfg]

Maybe if the id contains the expired date... Date.now() + "_" + v4.uuid()

Everytime you read a file, you must compare if(Date.parse(id.split('_')[0]) > Date.now()) and download it again