HAMMER 60G/Many: Mirroring, bug fixes

HAMMER 60G/Many: Mirroring, bug fixes

* Fix a bug in the mirror_tid propagation that could cause a delete_range
  iteration to get way way off.  B-Tree insertions were not calling
  hammer_cursor_inserted_element().

* Fix an assertion in the delete_range iteration, delete-visibility could
  cause the assert to trigger when it shouldn't.

* Add a new assertion testing that the iteration is still within the
  inode object to catch any remaining mirror_tid propagation bugs.

* Minor code documentation adjustments.

Reported-by: Gergo Szakal <bastyaelvtars@gmail.com>