Improve reconnection logic

[pm47]

When network access is lost (due to connectivity issues, or phone in sleep mode or...), we currently rely on periodic retries for reconnecting. That's not great because there can be a delay between the time when network is back on, and the time when we actually try to reconnect to our peers/electrum. It would be nice to use Android events (is there such a thing?) to tell eclair to reconnect as soon as network is back up.