swim: fix a bug with invalidation of round msg in fly

SWIM works in rounds, each split in steps. On step SWIM sends a
round message. The message is not changed between steps of one
round usually, and is cached so as not to rebuild it without
necessity. But when something changes in one member's attributes,
or in the member table, the message is invalidated to be rebuilt
on a next step. Invalidation resets the cached packet.

But it leads to a bug, when a round message is already scheduled
to be sent, however is not actually sent and invalidated in fly.
Such a message on a next EV_WRITE event will be sent as an empty
packet, which obviously makes no sense.

On the other hand it would be harmful to cancel the invalidated
packet if it is in fly, because during frequent changes the
instance will not send anything.

There are no a test case, because empty packets does not break
anything, but still they are useless. And, as it is said above,
such invalidation would prevent sending any round messages when
there are lots of updates.

Follow up for cf0ddeb8
(swim: keep encoded round message cached)