Chapter overview:

  * explains the channel graph, and how it's modified+verified

Relevant questions to answer:

  * Gossip announcements:
      - How does a peer announce a new channel to the network?
      - How do nodes verify a channel announcement? Why should they verify one in the first place?
      - How does a node control _how_ a payment is routed through its channel?
      - What knobs exist for a node to set in their channel updates?
      - How often are channel updates sent?
      - How does a node update its node in the channel graph? Do we we need to verify this?
      - How quickly does an update propagate?
      - What are "zombie" channels? Why do they matter?
  * Channel graph syncing:
      - What are the various ways a node can sync the channel graph?
      - Which is the most efficient?
      - What is the "gossip query" system?
      - Does a node need to keep up with all gossip updates? Does this change if they're a routing node or mobile client?
  * Protocol Extensions via Feature Bits and TLV:
      - How can the channel graph be upgraded using feature bits and TLV fields?
      - How does a receiver signal that they can accept MPP/AMP payments?