hammer - Add simple I/O optimization, collect stats on new deduplications

hammer - Add simple I/O optimization, collect stats on new deduplications

* Add a simple I/O optimization.  When dealing with CRC collisions scan
  the SHA sub-tree looking for the data_offset/data_len match, allowing
  us to avoid unnecessary I/O reads.

  This effects data which has already been de-duplicated, so it will improve
  dedup rescan times on the portion that has already been dedupped.

* Collect statistics on NEW deduplications during the pass and print them
  out in the final stats as well as in the ^T stats.