For some reason, using the shared hash causes a cryptographic incompatibility
when using Go 1.15. I noticed this during the development of Discovery v5.1
when I added test vector verification.
The go library commit that broke this is golang/go@97240d5, but the
way we used HKDF is slightly dodgy anyway and it's not a regression.
This adds an implementation of the current discovery v5 spec.
There is full integration with cmd/devp2p and enode.Iterator in this
version. In theory we could enable the new protocol as a replacement of
discovery v4 at any time. In practice, there will likely be a few more
changes to the spec and implementation before this can happen.