hammer2 - Stabilization pass, more flush refactoring

hammer2 - Stabilization pass, more flush refactoring

* Add voldata.inode_tid, separate inode TID allocations from
  transaction TID allocations in voldata.

* Rewrite the transaction management functions.

* Rewrite hammer2's filesystem sync code to reduce stalls.

* Keep track of a generation number on the hammer2_chain_core structure
  so the flush code can re-scan when it modifies elements within the
  flush transaction.

* Cleanup the duplication and delete-duplication code and hardlink handling.
  The delete-duplication code now properly tags delete_tid when a flush is
  delete-duplicating a chain which is deleted in the live view but is still
  valid in the flush view.

* Correct numerous bugs in tracking the modified/deleted state of
  a chain.

* Correct numerous flush bugs.

* Separate the mirror TID for the freemap chain from the volume chain.
  This will allow freemap updates to be delayed.

* Implement a more stringent algorithm to determine when CHAIN_MOVED
  can be cleared in chain->flags.

* Do a better job limiting the flush scan when concurrent modifying
  operations are occuring in large volumes.