This change re-introduces object expiration feature with some internal
differences from earlier implementation such as:

* Creation of zero-byte tracker object is performed directly on the
  mount point. Earlier HTTP request was sent to object server
  for the same. This incurred overhead of metadata creation for
  these zero-byte files which isn't necessarry as all required
  information is encoded in the path itself.
* Crawling of zero-byte tracker objects is done by the object
  expirer daemon itself and not container server.
* Deletion of tracker object is performed by the object expiration daemon
  directly on mount point.

Deletion of actual data object is not carried out by object expiration
daemon directly. The object expirer sends a DELETE request to object
server which deletes the actual object. This behaviour is not changed.

There is no change in behaviour in comparison with older implementation.
This is asserted by re-enabling existing functional tests without
any changes.

Change-Id: I01dc77cc4db3be3147d54e3aa2a19ed182498900
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Reviewed-on: http://review.gluster.org/13913
Reviewed-by: Thiago da Silva <thiago@redhat.com>
Tested-by: Thiago da Silva <thiago@redhat.com>