Process the combined LRU lists in LRU order

Process the combined LRU lists in LRU order

Only cleanup headers that are less than equal to the rbt's last_used
time.  Adjust the rbt's last_used time when the target cleaning was
not achieved to the oldest value of the remaining set of headers.

When updating delegating NS and glue records last_used was not being
updated when it should have been.

When adding zero TTL records to the tail of the LRU lists set
last_used to rbtdb->last_used + 1 rather than now.  This appoximately
preserves the lists LRU order.