hammer2 - Refactor flush

* Replace HAMMER2_CHAIN_SUBMODIFIED with core->update_tid.  SUBMODIFIED
  applies to chain->core, not to chain. Use a TID to track updates to
  make it easier for a flush to update records without messing up flush
  sequencing of chains being concurrently modified outside the flush's
  TID (that will be handled in the next flush).

* Make sure the DUPLICATED flag is set when duplicating a chain which
  has already been duplicated to another target.  This case is only during
  flushes and can occur when the flush races against concurrent updates
  which are not part of the flush.

* Refactor bioq flushing during a flush.  hammer2_vfs_sync now gives the
  bioq a window to operate using the flush's TID before the flush actually
  starts to flush.

* hammer2_chain_modify() retains the current allocation block if the TID
  does not cross a flush boundary.

* chain->bref.mirror_tid is now used to track flush progress and is compared
  against core->update_tid to determine when a flush is needed.

* Code cleanups.
test - Adjust vnodeinfo for system changes

* Make vnodeinfo in test/debug compile again.
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.
kernel - Improve panic handling

* Clear the gd_spinlocks counter when handling a panic.  This improves
  our chances of being able to obtain a crash dump
hammer1 - cleanup, minor bug fixes

* Cleanup pass, remove some dead code

* Minor bug fixes, add tokens around some paths that need them.

* Remove use of the master token in several paths that don't need it,
  improving concurrency.