Refactored S3Util data loader to accept parameters, e.g., number of buckets,
number of keys (min key, max key), object size. We now assume 2-levels
of hierarchy (bucketX/keyY) rather than the 3-levels we had before.
Keys are spread evenly across buckets.

Refactored S3Generator to generate requests for the 2-level bucket/key
hierarchy.

Modified the default S3 configuration.