Delete expired objects in smarter way.

When the expirer tries to delete customer objects, if it just walks through the
containers in order the deamon will tend to send DELETEs to the same container
highly concurrently. This will in turn create a lot of asyncs because of all
the concurrent deletes. If the deletes were spread out to multiple containers
it would improve performance and decrease the number of asyncs made.

Change-Id: I3d08118c197b7f18dd7e880bd5664508934ffd24