ck_ht: Enable aggressive re-use of tombstones.

Previously, a probe to an empty slot was necessary in order
to avoid hash table growth. As long as a tombstone is available,
re-use it. This prevents excessive growth on workloads involving
long bursts of writes (near 0.5 load factor) followed by long
bursts of deletes.
ck_ht: Remember to set cursor to NULL on probe end.