swim: push-pull dissemination

SWIM in the original paper says, that dissemination time of an
event is O(log(N)), where N is size of the cluster. It is true,
when both ping and ack messages carry dissemination and
anti-entropy. Before this patch it wasn't so - only regular
pings were carrying something.

After this patch the SWIM module has true exponential
dissemination speed.

Closes #4253