Add routing information gossip

[cdecker]

This is a tracking issue for the replacement of the IRC based node and channel announcement with a peer-to-peer based gossiping protocol.

• Add nested transport in protobuf-based protocol
• Add message for node and channel announcement
• Trigger timeout based announcements
• Reflect incoming announcements in routing table
• Staggered forward of incoming announcements
• Trigger channel event based announcements
• Dump channel_announcements and channel_updates to newly connected peers

I'm currently developing this locally, and will file a PR as soon as it is ready to review.