ck_hs: Re-use tombstones near load factor.

This is similar to the set of changes done to ck_ht.
In addition to this, a bug was caught were minimal
probe sequence was based on last rather than first
tombstone observed.

This hashset will now face long-running bursty
write -> delete -> write workloads much more effectively
and includes significant performance improvements for
delete heavy workloads (at least 2x measured).