Add support for Object Expiration feature

Preventing access to expired objects
------------------------------------
Re-enabled accepting X-Delete-At and X-Delete-After headers. During a GET on
an expired object, DiskFileExpired is raised by DiskFile class. This will
result in object-server returning HTTPNotFound (404) to the client.

Tracking objects to be deleted
------------------------------
Objects to be deleted are tracked using "tracker objects". These are PUT into
a special account(a volume, for now). These zero size "tracker objects" have
names that contain:
    * Expiration timestamp
    * Path of the actual object to be deleted

Deleting actual objects from GlusterFS volume
---------------------------------------------
The object-expirer daemon runs a pass once every X seconds. For every pass it
makes, it queries the special account for "tracker objects". Based on
(timestamp, path) present in name of "tracker objects", object-expirer then
deletes the actual object and the corresponding tracker object.

To run object-expirer forever:
    swift-init object-expirer start
To run just once:
    swift-object-expirer -o -v /etc/swift/object-expirer.conf

Caveat/Limitation: Object-expirer needs a separate account(volume) that
is not used by other services like gswauth. By default, this volume is
named "gsexpiring" and is configurable.

More info about object expiration:
http://docs.openstack.org/developer/swift/overview_expiring_objects.html

Change-Id: I876995bf4f16ef4bfdff901561e0558ecf1dc38f
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Reviewed-on: http://review.gluster.org/6891
Tested-by: Chetan Risbud <crisbud@redhat.com>
Reviewed-by: pushpesh sharma <psharma@redhat.com>
Tested-by: pushpesh sharma <psharma@redhat.com>
Reviewed-by: Chetan Risbud <crisbud@redhat.com>