hammer2 - Major restructuring, part 6/several

* Stabilization pass.

* Protect vfs_sync with a master lock to serialize operations, to protect
  the master volume header sequencing.  The related fsync scan will use the
  same transaction id.

* Fix a bug where races could cause SUBMODIFIED set in deep chains to not
  propagate back up to the root.

* Limit debug dumps on unmount.

* API adjustments.