HAMMER 39/Many: Parallel operations optimizations

HAMMER 39/Many: Parallel operations optimizations

* Implement a per-direct cache of new object IDs.  Up to 128 directories
  will be managed in LRU fashion.  The cached provides a pool of object
  IDs to better localize the object ids of files created in a directory,
  so parallel operations on the filesystem do not create a fragmented
  object id space.

* Cache numerous fields in the root volume's header to avoid creating
  undo records for them, creatly improving

  (ultimately we can sync an undo space representing the volume header
  using a direct comparison mechanic but for now we assume the write of
  the volume header to be atomic).

* Implement a zone limit for the blockmap which newfs_hammer can install.
  The blockmap zones have an ultimate limit of 2^60 bytes, or around
  one million terrabytes.  If you create a 100G filesystem there is no
  reason to let the blockmap iterate over its entire range as that would
  result in a lot of fragmentation and blockmap overhead.  By default
  newfs_hammer sets the zone limit to 100x the size of the filesystem.

* Fix a bug in the crash recovery code.  Do not sync newly added inodes
  once the flusher is running, otherwise the volume header can get out
  of sync.  Just create a dummy marker structure and move it to the tail
  of the inode flush_list when the flush starts, and stop when we hit it.

* Adjust hammer_vfs_sync() to sync twice.  The second sync is needed to
  update the volume header's undo fifo indices, otherwise HAMMER will
  believe that it must undo the last fully synchronized flush.