Review & rework & clean-up RLPx auth & ecies code

[kdeme]

After fixing the crashes in #558 and looking at more detail at that code I think we should:

• Get rid of any of the code that is there to be compatible with pre-EIP8. Geth this did this May last year here ethereum/go-ethereum@7194c84. That should make the code easier to begin with.
• Go fully through the auth and ecies code as there might be more invalid accesses possible.
• I think some of the size checks are not really on the right place / redundant now in ecies. Maybe not. Anyway, to be reviewed.