HAMMER 28A/many: Translation and write performance optimizations

HAMMER 28A/many: Translation and write performance optimizations

* Cache a buffer's virtual (zoneX->zone2) translation to avoid unnecessary
  re-translations when a hot-cached buffer is available.

* Use the sequential heuristic provided by the kernel to detect sequential
  writes and call bawrite() instead bdwrite() in such cases.  This
  prevents the buffer cache from hitting slow fallback cases and
  completely solves the write performance issue.

  Note that the undo fifo is not yet in place and performance will drop
  slightly once it is, but what you see now is basically what you're gonna
  get in the final.

* Performance is now on par with UFS.