HAMMER 56A/Many: Performance tuning - MEDIA STRUCTURES CHANGED!

HAMMER 56A/Many: Performance tuning - MEDIA STRUCTURES CHANGED!

* MEDIA CHANGE: The blockmaps have been folded into the freemap.  Allocations
  are now made directly out of the freemap.  More work is expected here.

  The blockmaps are still used to sequence allocations, but no block
  number translation is required any more.  This didn't improve performance
  much but it will make it easier for future optimizations to localize
  allocations.

* PERFORMANCE: Removed the holes recording code.  Another commit will
  soon take over the functionality.

* PERFORMANCE: The flusher's slave threads now collect a number of inodes
  into a batch before starting their work, in an attempt to reduce
  deadlocks between slave threads from adjacent inodes.

* PERFORMANCE: B-Tree positional caching now works much better, greatly
  reducing the cpu overhead when accessing the filesystem.

* PERFORMANCE: Added a write-append optimization.  Do not do a
  lookup/iteration to locate records being overwritten when no such
  records should exist.  This cuts the cpu overhead of write-append
  flushes in half.

* PERFORMANCE: Add a vfs.hammer.write_mode sysctl feature to test out
  two different ways of queueing write I/O's.

* Added B-Tree statistics (hammer bstats 1).