[9.16.49] Remove expired rdataset headers from the heap

It was discovered that an expired header could sit on top of the heap a little longer than desireable. Remove expired headers (headers with rdh_ttl set to 0) from the heap completely, so they don't block the next TTL-based cleaning.

(cherry picked from commit a9383e4b) (cherry picked from commit abe080d1)

Closes #4591 (closed)

Backport of !8754 (merged)