hammer2 - cleanup, stabilization pass

hammer2 - cleanup, stabilization pass

* MOVED -> SUBMODIFIED was not being set in the parent chain under some
  circumstances.

* Optimize I/O a bit more, there's no point dirtying a chain for a blockref
  update until all sub-chains are clean (at least for now).

* Additional chain locking around certain tests and propagations.

* Numerous fixes to hammer_chain_flush(), sync, and unmount

* Properly destroy the chains for disconnected inodes when the inodes are
  reclaimed.

* Code cleanup.