In order to not swamp bootstrap nodes with connection attempts, a node that is well connected should leave bootstrap nodes alone.

• Add a new peer message to communicate the own bootstrap status
• Persistently record the bootstrap status of peers
• Stop connecting to bootstrap nodes if there are enough peers

Some open design decisions:

• What does “well connected” mean? Should this be a CLI argument? A hardcoded number, like 6 peers? A ratio, like 80% of max_num_peers?
• …

(This is the other side of the #411-coin.)