Turns out the DiscV5 code relies heavily on the presence of ENR
records at the moment, so we cannot drive it with ENodes. @kdeme
is working on refactoring that will relax these requirements.
* The bootstrap_nodes.txt file in the node's data dir is now optional
* Log more data on start-up
* Use the latest ENR APIs
* Fix simulation build errors
We no longer discriminate between ENR, MultiAddress or ENode
bootstrap records (all of them are remapped to ENodes).
The discovery loop will stochastically try to reconnect to
accidentally disconnected nodes.
Zahary Karadjov