Remove expired rdataset headers from the heap

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 a9383e4b95256a65f9f05e64a79b086a9a1ed035)
(cherry picked from commit abe080d16eac4f5dcca62cb08bd9ca2f82bdaa2b)