virtual directory colocation

[gregjhogan]

Support placing items with the same prefix before the last slash in the key on the same volume server(s)

For example, the following keys will end up on the same volumes:
/a/b/c/1.txt
/a/b/c/2.txt

The benefit is if you lose one or more drives you are guaranteed to have complete virtual folders remaining (all files in a virtual folder path still exist).

Note that since virtual folder paths are being distributed randomly instead of files, the virtual folder paths need to all be of similar size to keep volumes balanced (volume disks all fill up at an equal rate)