Previously, a thousand-item multi-delete request would consider each
object to delete serially, and not start trying to delete one until the
previous was deleted (or hit an error).
Now, allow operators to configure a concurrency factor to allow multiple
deletes at the same time.
Default the concurrency to 2, like we did for slo and bulk.
See also: http://lists.openstack.org/pipermail/openstack-dev/2016-May/095737.html
Change-Id: If235931635094b7251e147d79c8b7daa10cdcb3d
Related-Change: I128374d74a4cef7a479b221fd15eec785cc4694a
00be3f595e