HAMMER 54B/Many: Performance tuning.

HAMMER 54B/Many: Performance tuning.

* Fix a major performance issue in the UNDO generation code.  The code
  was calling hammer_bread() instead of hammer_bnew() for 'new' undo buffers,
  meaning it was doing a read-modify-write on the disk instead of just a
  write.

  This fix results in a MAJOR improvement in performance across the board.

* Replace the only lockmgr lock in the module with a hammer_lock.

* Tweak hammer_inode_waitreclaims().  This will probably need even more
  tweaking as time passes.