HAMMER 35/many: Stabilization pass, cleanups

HAMMER 35/many: Stabilization pass, cleanups

* Fix a buffer load race which could result in an assertion or panic
  related to a referenced HAMMER buffer with a NULL bp.  The problem was
  that the loading flag must be used when releasing the buffer as well
  as when acquiring the buffer.  Change the loading flag to a loading
  count.

* Do not lose flush requests.  The flush request now stays flagged until
  the buffer is able to be flushed.

* Fix stale blockmap offsets cached in hammer_buffer.  Clear the cached
  offset when freeing a big block from the blockmap.  NOTE: We do not yet
  try to index buffers based on the blockmap offset but we should.

* Remove the old write ordering code in preparation for redoing the algorithm.

* General code cleanups.