ck_hs: Add ck_hs_put_unique function for fast-path unique inserts.

This function allows for faster insertions into tombstone-heavy
probe sequences by short-circuiting on tombstones rather than
continuing to probe. The user must already guarantee that the
entry being inserted is unique. If a non-unique key is inserted
with this operation, undefined behavior will result.
regressions: Coverage for ck_hs_put_unique.
ck_hs: Saturate to quadratic probe from double hashing.

This should primarily affect specific delete-heavy workloads.
ck_internal: Fix bsf behavior.