sbin/hammer: Implement checkmap collects using rbtree

sbin/hammer: Implement checkmap collects using rbtree

- Replace hash table for collects using rbtree.

- Using hash table can't dump collects' -v info in the right order
  since hash index value and collect entries linked to hash table
  entries do not reflect collect::phys_offset value.

- If the amount of filesystem data is less than approximately 4TB
  nothing will change because there is only one collect entry,
  which makes no difference whether the data structure is rbtree
  or hash table.