ck_hs: Add atomic swap operation (requires existence).
ck_hs: Existence requires generation check.

It is possible for a defragmenting set or swap operation
to set a tombstone. If the probe sequence does not encounter
an empty slot and hits maximum write-side probe limit first
for it to fail to reprobe defragmenting store.
ck_hs: Prototype for new ck_hs_fas operation.
regressions: Testing of ck_hs_fas functionality.