Forklift the DiskFile interface into it's own module

 * new module swift.obj.diskfile

I parameterized two constants from obj.server into the DiskFile's __init__

 * DATADIR -> obj_dir
 * DISALLOWED_HEADERS -> disallowed_metadata_keys

I'm not sure if this is the right long term abstraction but for now it avoids
circular imports.

Change-Id: I3962202c07c4b2fbfc26f9776c8a5c96292ae199