swim: introduce routing

Before the patch SWIM packets were being sent quite
straightforward from one instance to another with transparent
routing on Internet Level of TCP/IP. But the SWIM paper
describes last yet not implemented component - suspicion
mechanism.

So as not to overload this message with suspicion details it is
enough to say that it makes possible sending a packet through an
intermediate SWIM instance, not directly.

This commit extends the SWIM protocol with a new transport-level
section named 'routing'. It allows to send indirect SWIM messages
transparently via packet forwarding implemented fully inside
transportation component, in swim_io.c.

Part of #3234