This PR adds support for sending back node announcements when replying to channel range queries:
- when explicitly requested (bit is set in the optional query flag)
- when query flags are not used and a channel announcement is sent (as per the BOLTs)

A new configuration option `request-node-announcements` has been added in the `router` section. If set to true, we
will request node announcements when we receive a channel id (through channel range queries) that we don't know of.
This is a setting that we will probably turn off on mobile devices.

There is now more work to do.