Don't preserve cache entries if new TTL is smaller than existing

Don't preserve cache entries if new TTL is smaller than existing

Under certain circumstances, cache entries with equivalent rdataset
might not get replaced.  Previously such entry would get preserved
regardless of the new TTL and expire time on the existing header would
get updated when the expire time was less than the expire time on the
existing header.  Change the logic to preserve the existing header only
if the new expire time is larger than the existing one and replace the
existing cache entry when the new expire time is less than the existing
one.

Co-authored-by: Jinmei Tatuya <jtatuya@infoblox.com>