This change moves the RLPx protocol implementation into a separate package,
p2p/rlpx. The new package can be used to establish RLPx connections for
protocol testing purposes.

Co-authored-by: Felix Lange <fjl@twurst.com>

I forgot to update one instance of "go-ethereum" in commit 3f047be5.

All code outside of cmd/ is licensed as LGPL. The headers
now reflect this by calling the whole work "the go-ethereum library".

Message encoding functions have been renamed to catch any uses.
The switch to the new encoder can cause subtle incompatibilities.
If there are any users outside of our tree, they will at least be
alerted that there was a change.

NewMsg no longer exists. The replacements for EncodeMsg are called
Send and SendItems.

With RLPx frames, the message code is contained in the
frame and is no longer part of the encoded data.

EncodeMsg, Msg.Decode have been updated to match.
Code that decodes RLP directly from Msg.Payload will need
to change.

Overview of changes:

- ClientIdentity has been removed, use discover.NodeID
- Server now requires a private key to be set (instead of public key)
- Server performs the encryption handshake before launching Peer
- Dial logic takes peers from discover table
- Encryption handshake code has been cleaned up a bit
- baseProtocol is gone because we don't exchange peers anymore
- Some parts of baseProtocol have moved into Peer instead

...and make it a top-level function instead.

The original idea behind having EncodeMsg in the interface was that
implementations might be able to encode RLP data to their underlying
writer directly instead of buffering the encoded data. The encoder
will buffer anyway, so that doesn't matter anymore.

Given the recent problems with EncodeMsg (copy-pasted implementation
bug) I'd rather implement once, correctly.