hammer2 - xop stabilization - part 2/many

hammer2 - xop stabilization - part 2/many

* The xop mechanics do a quorum calculation, and will also do quorum
  calculations for the inode cluster (not yet in).  This calculation is
  primarily based on the bref.modify_tid.

  Each backend XOP now needs its own modify_tid to properly distinguish
  between and eventually serialize dependent backend ops.

* Implement discrete modify_tid (mtid) reservations in
  hammer2_xop_alloc().  Remove the old modify_tid reservation code that only
  incremented it on a per-flush basis.

* The flush code will acquire a unique modify_tid which is greater than all
  prior modify_tids in stage 2 (after it clears PREFLUSH).

* The hammer2_xop_alloc() code is too messy returning the union structure,
  return a void * instead so the caller can just assign it to the correct
  sub-union structure.