Print stats/diagnostics on system signal

[raulk]

For debugging the state of the daemon, it could be useful to add a hook to catch SIGUSR2 (as we plan to use SIGUSR1 for config reload) that prints statistics/diagnostics to stdout/stderr.

We can output at least:

• dht.RoutingTable.Print()
• active connections with peers.
• active streams with peers, including the protocol id.
• pubsub subscriptions.

For now, the DHT routing table would suffice, as we actually do need it kind of immediately.

[vyzo]

First pass, we dump the routing table on SIGUSR1.

[vyzo]

#101 implemented peer and stream dumps.