swim: disseminate event for log(cluster_size) steps

Before the patch there was a problem of events and anti-entropy
starvation, when a cluster generates so many events, that they
consume the whole UDP packet. A packet fits up to 26 events. If
during the event storm something important happens, that event is
likely to be lost, and not disseminated until the storm is over.

Sadly, there is no way to prevent a storm, but it can be made
much shorter. For that the patch makes TTD of events logarithmic
instead of linear of cluster size.

According to the SWIM paper and to experiments the logarithm is
really enough. Linear TTD was a redundant overkill.

When events live shorter, it does not solve a problem of the
events starvation - still some of them can be lost in case of a
storm. But it frees some space for anti-entropy, which can finish
dissemination of lost events.

Experiments in a simulation of a cluster with 100 nodes showed,
that a failure dissemination happened in ~110 steps if there is
a storm. Basically, no dissemination at all.

After the patch it is ~20 steps. So it is logarithmic as it
should be, although with a bigger constant than without a storm.

Part of #4253