As reported at bug/1546067, expirer might accidentally deletes an object
which is created after x-delete-at timestamp. This is because expirer
sends a request with "X-Timestamp: <current_timestamp>" and tombstone
is named as <requested_x_timestamp>.ts so if object creation time is
between x-delete-at and expirer's DELETE request x-timestamp, the object
might be hidden by tombstone.
This possibility can be simply removed if the value of x-timestamp which
an expirer sends is the same timestamp as x-delete-at of an actual object.
Namely, expirer pretends to delete an object at the time an user really
wants to delete it.
Change-Id: I53e343f4e73b0b1c4ced9a3bc054541473d26cf8
Closes-Bug: #1546067
22933f5b55