In dns_slabheader_t structure, change .ttl to .expire

In dns_slabheader_t structure, change .ttl to .expire

The old name was misleading as it never meant time-to-live, e.g. number
of seconds from now when the header should expire.  The true meaning was
an expiration time e.g. now + ttl.  This was the original design bug
that caused the slip when we assigned header->ttl to rdataset->ttl.
Because the name was matching, nobody has questioned the correctness of
the code both during the MR review and during the numerous re-reviews
when we were searching for the cause of the 54 year TTL.