filed: Add support for Pithos object migration

Pithos places its objects under the hexlified hash of the contents. It also
creates the dir hierarchy according to the first 6 characters of the object
name. In contrast, filed hashes the object name to get 6 random characters for
the dir hierarchy. In this commit, we introduce a fallback path for filed to
find objects created by Pithos and move them to the directory expected by filed.
filed: Split dir creation

Split dir creation into a different function. Also avoid looping around mkdir.
filed: Do not create dirs when doing read I/O
filed: Add migrate flag

Add a boolean migrate flag, to enable migrations of pithos objects to their new
locations.