hammer2 - xop stabilization - part 1/many

hammer2 - xop stabilization - part 1/many

* Create XOP threads on-demand, they are needed sometimes even on PFSs
  which don't normally need it.

* For spmp roots, count PFSTYPE_SUPROOT as a master to simplify the cluster
  validation code.

* Allow hammer2_inode_get() to only repoint part of an inode's cluster.

* Stabilize the slave synchronizer (a little, requires more).

* Remove more dead code.