HAMMER 40F/Many: UNDO cleanup & stabilization.

HAMMER 40F/Many: UNDO cleanup & stabilization.

* Properly classify UNDO zone buffers so they are flushed at the correct
  point in time.

* Minor rewrite of the code tracking the UNDO demark for the next flush.

* Introduce a considerably better backend flushing activation algorithm
  to avoid single-buffer flushes.

* Put a lock around the freemap allocator.