JCLOUDS-510 Delete objects in a container efficiently.

The existing approach for deleting objects in a container suffers
from a head-of-line blocking problem. This commit implements a better
scheme which does not have that problem. This scheme uses a counting
semaphore for making sure that a certain number of futures are
issued in parallel. As each of these futures is completed, one
permit of the semaphore is released.

Added unit tests for testing this new scheme.