relay service -- streams across peers

this is the leadup into NAT traversal.
note: doesn't work yet. hangs the test.
ipfsnet -> swarmnet

swarmnet is a better name for the package, because
it's just a Network implemented with a Swarm.
(ipfsnet will be something slightly different).
add relaying to mocknet

These services should all be added separately, in a function
that is Network impl independent. The interfaces need to be
massaged a bit (split inet.Network in two), so will do when
needed. For now this is fine.