Collections: disk/data size delete->delete is increasing the counter

Spotted whilst test MB-45144 the following sequence results in an increase of the counter when it should result in no change.

• store(k1, "value")
• flush
  • this is an insert, so counter increases e.g. size:32 (key+value+meta)
• delete(k1)
• flush
  • this is a delete (but really just a new write of k1 but with no value) so counter decreases, as we account tombstones (key+meta remains) size:18

Next another delete, this requires a store/delete before flush, de-dup means we only flush the delete, but a store is required so that the delete doesn't fail with "no-key".

• store(k1, "value")
• delete(k1)
• flush

So we only now flush a delete, the disk is logically unchanged but the stat code treats this as a insert and size goes up, i.e. size becomes 18 + 18