IPLD Resolver: Eclipse attack protection

[aakoshh]

#64 added the mechanism of adding the addresses from Identify events to the k-table, otherwise incoming connections (presumably from ephemeral addresses) to Kademlia were treated as unroutable.

The problem with that solution is that a group of malicious nodes can quickly identify themselves to the node to stuff its k-table full of working but adversarial addresses, and crowd out legitimate peers, eclipsing the node.

The solution is to make sure we bootstrap first and add peers which we discovered on our own, before we accept self-identified records. The identify only runs once every 5 minutes by default, so we should cache a limited number of them until the bootstrapping is over.

See https://eprint.iacr.org/2018/236

[aakoshh]

Similar to input-output-hk/scalanet#108