flistd: add cache directory indirection to avoid large directory size

[maxux]

@delandtj noticed some large usage of flist cache directory, like 7M directory size, which is not optimal at all for storage.
An easy workaround to fix that is using indirection like cache/fa/db/abcd00123... (for hash fadbabcd00123...)
An alternative could also be fa/db/fadbabcd00123 in order to keep filename as full hash.

[zaibon]

moved this issue from https://github.com/threefoldtech/zos to this repo since the organization of the files of the cache is management by 0-fs itself, not 0-OS