implement connectivity change in peer discovery

this makes peer discovery pub/sub on connectivity change, it aims to
make peer discovery more energy efficient.

also, stop echo'ing.

TODO:
in its current implementation however, peer_discovery operates in the
dark; it doesn't know routes available to it (v4 vs v6); nor does it
know of the state of the link(s) (UP vs. DOWN) and therefore it merely
tries to, on connectivity change, (re)pub/sub on previous ip version
routes.

IDEA:
open an interface to kernel so that peer discovery, and perhaps others,
can learn about the routes available to them.

don't leave

add query() and direct publish(endpoint)

check if peer discovery was request (run and compile time)

add try/catch routine to DomainPeerDiscovery's constructor