devp2p: drop pre-EIP8 support, fix snappy negotiation (#758)

Support for previous versions was
[removed](ethereum/go-ethereum@7194c84)
from geth in 2021 after other clients had migrated - should be safe to
remove here also.

* fix snappy detection - it should use the hello version, not the RLPx
handshake
* simplify generation of auth messages
* pad 100-300 bytes like spec suggests

TODO: error handling is all over the place - will be addressed in a
follow-up PR