hammer2 - Refactor frontend part 9/many

hammer2 - Refactor frontend part 9/many

* Create initial frontend/backend XOP infrastructure.

  frontend:
	hammer2_xop_alloc()
	hammer2_xop_start()
	...  hammer2_xop_collect() loop ...
	hammer2_xop_retire(xop, HAMMER2_XOPMASK_VOP)

  backend:
	(backend is called with the shared xop structure in separate
	 backend threads for each node belonging to the cluster appropriate
	 for the operation).

	... issue chain calls as needed ...
	... hammer2_xop_feed() ...		(feed chains back to frontend)
	hammer2_xop_feed(NULL)			(feed NULL chain)
	hammer2_xop_retire(xop, 1U << clindex)

  The XOP contains a FIFO, allowing the backend to pipeline results when
  appropriate (e.g. readdir).  If a sequence of results are expected, the
  backend should finish with a NULL chain.  If not, the backend can just feed
  back whatever is expected.  Often this will just be the chain representing
  the inode.

  The frontend calls hammer2_xop_collect() to collect results from all the
  backend nodes.  The collect function handles quorum validation and
  consolidates the results from a sufficient number of cluster nodes into
  a single result for the frontend.

* The frontend can disconnect from the operation at any time in order to
  be able to return a result, even if backend elements are still running.
  This typically occurs when a sufficient number of nodes in the cluster
  have responded to validate the quorum.

  This also allows backend nodes to stall indefinitely without stalling the
  frontend.

* Because frontend concurrency is lost due to the bulk of the work being
  done by the backend, the hammer2 mount code will allocate ~16 or so
  work threads per node to distribute potentially many frontend operations.

* Most frontend operations use existing cache layers to retains frontend
  concurrency.  Specifically, inode meta-data access and modifications,
  and logical buffer cache operations (when cached), and cached vnodes
  via the namecache.  If the cache is not available, operations will
  wind up using the VOP/XOP infrastructure, including buffer-cache
  strategy routines (in an upcoming commit).

* Implement readdir() using the new infrastructure as an initial test.

* Fix an ip->meta synchronization bug related to hardlinks that was
  introduced by the ip->meta local copy work.