add a mutable-file cache

[warner]

If the mutable-file retrieval process were allowed to keep an on-disk cache (indexed by SI+roothash, populated by either publish or retrieve), then a lot of directory-traversal operations would run a lot faster.

The cache could store plaintext, or ciphertext (or, if/when we implement deep-traversal caps, it could contain the intermediate form). For a node that runs on behalf of a single user, the plaintext cache would be fastest. For a webapi node that works for multiple users, I wouldn't feel comfortable holding on to the plaintext for any longer than necessary (i.e. we want to maintain forward secrecy), so if a cache still seemed useful then I'd want it to be just a ciphertext cache.

The cache should only store one version per SI, so once we publish or discover a different roothash, that should replace the old cache entry. The cache should have a bounded size, with a random-discard policy. Of course we need some efficient way to manage that size (doing 'du -s' on the cache directory would be slow for a large cache, either we should keep the cache from getting that large or do something more clever).

It would probably be enough to declare that the cache is implemented as a single directory, with one file per SI, and each file contains the base32-encoded roothash + newline + plaintext/ciphertext . The size bound is imposed by limiting the number of files in this directory, and it is counted at startup.