Refactor DiskFile hiding quarantine adding reader

This set of refactorings is proposed as the last step before
officiallly declaring the DiskFile class, and its helpers, to be the
backend API servicing the Object Server REST API implementation code.
The goal is to ensure that the REST API layer in the object server
does not have any access to internal knowledge of how the DiskFile
class is implemented.

We hide the need for the REST API layer to know how and where
quarantining needs to be performed. There are now two places it is
checked internally, on open() where we verify the content-length,
name, and x-timestamp metadata, and in the reader on close where the
etag metadata is checked if the entire file was read.

We add a reader class to allow implementations to isolate the WSGI
handling code for that specific environment (it is used no-where else
in the REST APIs). This simplifies the caller's code to just use a
"with" statement once open to avoid multiple points where close needs
to be called.

For a full historical comparison, including the usage patterns see:
https://gist.github.com/portante/5488238

(as of master 8a255a3 Merge
 "Refactor finalize_put as an    This Commit
 object method")
--------------------------------+------------------------------------
                                 DiskFileManager(conf)

                                   Methods:
                                     .pickle_async_update()
                                     .get_diskfile()
                                     .get_hashes()

                                   Attributes:
                                     .devices
                                     .logger
                                     .disk_chunk_size
                                     .keep_cache_size
                                     .bytes_per_sync

DiskFile(a,c,o,keep_data_fp=)    DiskFile(a,c,o)

  Methods:                         Methods:
   *.__iter__()
    .close(verify_file=)
    .is_deleted()
    .is_expired()
    .quarantine()
    .get_data_file_size()
                                     .open()
                                     .read_metadata()
    .create()                        .create()
                                     .write_metadata()
    .delete()                        .delete()

  Attributes:                      Attributes:
    .quarantined_dir
    .keep_cache
    .metadata
                                *DiskFileReader()

                                   Methods:
                                     .__iter__()
                                     .close()

                                   Attributes:
                                    +.was_quarantined

DiskWriter()                     DiskFileWriter()

  Methods:                         Methods:
    .write()                         .write()
    .put()                           .put()

* Note that the DiskFile class   * Note that the DiskReader() object
  implements all the methods       returned by the
  necessary for a WSGI app         DiskFileOpened.reader() method
  iterator                         implements all the methods
                                   necessary for a WSGI app iterator

                                 + Note that if the auditor is
                                   refactored to not use the DiskFile
                                   class, see
                                   https://review.openstack.org/44787
                                   then we don't need the
                                   was_quarantined attribute

Change-Id: Iefbbe7aada972b1ff2fc9fd5492646ab01fa192c
Signed-off-by: Peter Portante <peter.portante@redhat.com>