HAMMER VFS - Reduce stalls during bulk file operations

HAMMER VFS - Reduce stalls during bulk file operations

* Track modifying inode operations on a per-PID basis (loosely) and
  call hammer_inode_wait_reclaims() earlier for those pids.

  The algorithm selects a wait point based on the process's perceived
  contribution to the inode load.  The greater the contribution, the
  more readily we stall the process in order to wait for related reclaims
  to process.

  Processes with lower loads have higher reclaim points and do not stall
  as readily as they did before.

* Remove waitreclaims calls based on B-Tree scans.  I'm not sure why I had
  this in there but it was creating an excessive number of unnecessary
  stalls, so if any problems crop up I'll have to find another way to deal
  with them.

* These changes (particularly the first) should reduce unnecessary stalls
  for the programs not doing heavy inode operations.  Hopefully that means
  rm -rf and tar extractions will not have as quite the detrimental effect
  on other processes as they did before.