hammer2 - refactor some chain bits, stabilization

hammer2 - refactor some chain bits, stabilization

* Refactor the hammer2_chain_t MODIFIED and UPDATE flags to not
  reference the chain.

* Automatically flag DESTROY on a chain on last-drop if chain->parent is
  NULL, and automatically clear UPDATE and try to clear MODIFY.  Add the
  chain to the delayed-flush queue if MODIFIED cannot be cleared.

* Fix a flags bug in hammer2_chain_hardlink_find() which could deadlock
  competing threads.

* The collect code now aggregates errors and the cluster check code now
  also aggregates errors.  This allows feeders to feed an error code to
  the collector.

* Fix rmdir and rename to disallow removal of non-empty directories.

* hammer2_inode_xop_flush() now proceeds with a flush if hmp->flushq
  is not empty, even if the chain is not marked for flushing.  This
  still needs some work.

* Add a sideq for dirty inodes that might have to be flushed.  Used for
  unlinked dirty inodes which might still be open.

* Call hammer2_inode_modify() whenever possible instead of setting the
  HAMMER2_INODE_MODIFIED flag manually.

* Test for chain leaks against blogbench, fsx, and fsstress.