swim: introduce dissemination component

Dissemination components broadcasts events about member status
updates. When any member attribute is updated (incarnation,
status, UUID, address), the member stands into an event queue.
Members from the queue are encoded into each round step message
with a higher priority and before anti-entropy section.

It means, then even if a cluster consists of hundreds of members
and one of them was updated on one of instances, this update will
be disseminated regardless of whether this memeber is encoded
into anti-entropy section or not. It drastically speeds events
dissemination up, according to the SWIM paper, and is noticed in
the tests.

Part of #3234