ck_hs: Add improved support for delete-heavy workloads.

This borrows from a technique described by Purcell and Harris
in "Non-blocking hashtables with open addressing" technical report.
Essentially, every slot will have an associated local probe maxim,
including tombstones. Highly aggressive workloads may still require
occassional garbage collection.
regressions/ck_hs: Coverage for delete mode.
ck_hs: Bound delete to local maxim.
regressions/ck_hs: Serial benchmark coverage for DELETE mode.
Merge branch 'master' of git.concurrencykit.org:ck