Remove keep_data_fp argument from DiskFile constructor

All access to the data_file fp for DiskFile is moved after the new "open"
method.  This prepares to move some additional smarts into DiskFile and reduce
the surface area of the abstraction and the exposure of the underlying
implementation in the object-server.

Future work:

 * Consolidate put_metadata to DiskWriter
 * Add public "update_metdata" method to DiskFile
 * Create DiskReader class to gather all access of methods under "open"

Change-Id: I4de2f265bf099a810c5f1c14b5278d89bd0b382d
Merge "Remove keep_data_fp argument from DiskFile constructor"