[MEDIUM] only consider slow checks when looking for the common interval

[MEDIUM] only consider slow checks when looking for the common interval

When one server in one backend has a very low check interval, it imposes
its value as the minimal interval, causing all other servers to start
their checks close to each other, thus partially voiding the benefits of
the spread checks.

The solution consists in ignoring intervals lower than a given value
(SRV_CHK_INTER_THRES = 1000 ms) when computing the minimal interval,
and then assigning them a start date relative to their own interval
and not the global one.

With this change, the checks distribution clearly looks better.