HAMMER Utility - Refactor the histogram code for mirror-stream.

HAMMER Utility - Refactor the histogram code for mirror-stream.

* Refactor the histogram code.  This code is responsible for breaking
  down a large initial mirroring stream into smaller chunks so the
  transaction id can be synced more often.  This way if the stream
  is interrupted it can be restarted at a more recent point instead
  of having to restart further back (or at the beginning).

* Add -S splitsize (default 100MB) to specify the desired breakdown
  for the histogram.