BUG/MEDIUM: checks: make sure to always apply offsets to now_ms in expiration

BUG/MEDIUM: checks: make sure to always apply offsets to now_ms in expiration

Now_ms can be zero nowadays, so it's not suitable for direct assignment to
t->expire, as there's a risk that the timer never wakes up once assigned
(TICK_ETERNITY). Let's use tick_add(now_ms, 0) for an immediate wakeup
instead. The impact here might be health checks suddenly stopping.

This should be backported where it applies.

(cherry picked from commit 2f287f14f355e734e512732e35aebf993d000792)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>