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>