This avoids a proliferation of hash directories when using new-style
chunking, and should improve performance since chunks are accessed
in sequence and so should have a common locality.
Of course, when a chunked key is encrypted, its hash directories have no
relation to the parent key.
This commit was sponsored by Christian Kellermann.
Added new fields for chunk number, and chunk size. These will not appear
in normal keys ever, but will be used for chunked data stored on special
remotes.
This commit was sponsored by Jouni K Seppanen.
