Use atomics for the zone limit and sleeper count. This relies on the
sleepq to serialize sleepers. This patch retains the existing sleep/wakeup
paradigm to limit 'thundering herd' wakeups. It resolves a missing wakeup
in one case but otherwise should be bug for bug compatible. In particular,
there are still various races surrounding adjusting the limit via sysctl
that are now documented.

Discussed with: markj
Reviewed by: rlibby
Differential Revision: https://reviews.freebsd.org/D22827