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Changes between Initial Version and Version 1 of Ticket #465

initial	v1
6	6
7	7	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.
	8
	9	Note that because the cache is indexed by (SI,roothash), it is an accurate cache: a servermap-update is always performed (incurring one round-trip), and only the retrieve phase is bypassed upon a cache hit. This only helps improve retrieves of directories that are large enough to not fit in the initial read that the servermap-update performs (since there is already a small share-cache that holds these reads), which probably means 6 children or more per directory. These not-so-small directories could be fetched in a single round-trip instead of two RTT.
	10
	11	If we allowed the cache to be indexed by just SI (or if we were to introduce a separate cache that mapped from SI to somewhat-current-roothash), it would be an inaccurate cache, implementing a tradeoff between up-to-dateness and performance. In this mode, the node would be allowed to cache the state of a directory for some amount of time. We'd get zero-RTT retrieves for some directories, but we'd also run the risk of not noticing updates that someone else had made.