Fix race condition in _delete_container() where all elements of
object_queue have been removed, but the last one (per thread) may not
have actually been deleted yet when the container deletion thread calls
conn.delete_container(container). Fixes bug 1032879.
Improves container deletion throughput by immediately deleting
containers with no objects instead of waiting for all pending object
deletes to complete. Fixes bug 1032878.
Change-Id: I404229a4c608995294e0ada77724ac8afe8d6f3c