HAMMER - Rework write pipelining

HAMMER - Rework write pipelining

* Rework write pipelining so it is based on pending direct writes on an
  inode-by-inode basis.  ip->rsv_recs and hmp->rsv_recs are now
  decremented after the direct has completed rather then when the sync
  code has processed the record.

  This fixes serious buffer cache overloading when doing linear writes.

* Implement write clustering or bawrite() calls based on a filesystem
  block getting filled up instead of relying on the buffer cache's bdwrite()
  to keep ahead of the mark.

* vfs.hammer.cluster_enable now effects both read and write clustering.