Faster swift-dispersion-populate

- Makes swift-dispersion-populate a bit faster when using a larger
  dispersion_coverage with a larger part_power.
- Adds option to only run population for container OR objects
- Adds option to let you resume population at given point (useful if you
  need to resume population after a previous run error'd out or the
  like) by specifying which suffix to start at.

The original populate just randomly used uuid4().hex as a suffix on the
container/object names until all the partition's required where covered.
This isn't a big deal if you're only doing 1% coverage on a ring with a
small part power but takes ages if you're doing 100% on a larger ring.

Change-Id: I52f890a774412c1d6179f12db9081aedc58b6bc2
Merge "Faster swift-dispersion-populate"