This change significantly improves the performance of RLPx message reads
and writes. In the previous implementation, reading and writing of
message frames performed multiple reads and writes on the underlying
network connection, and allocated a new []byte buffer for every read.

In the new implementation, reads and writes re-use buffers, and perform
much fewer system calls on the underlying connection. This doubles the
theoretically achievable throughput on a single connection, as shown by
the benchmark result:

    name             old speed      new speed       delta
    Throughput-8     70.3MB/s ± 0%  155.4MB/s ± 0%  +121.11%  (p=0.000 n=9+8)

The change also removes support for the legacy, pre-EIP-8 handshake encoding.
As of May 2021, no actively maintained client sends this format.

This PR contains a minor optimization in derivesha, by exposing the RLP
int-encoding and making use of it to write integers directly to a
buffer (an RLP integer is known to never require more than 9 bytes
total). rlp.AppendUint64 might be useful in other places too.

The code assumes, just as before, that the hasher (a trie) will copy the
key internally, which it does when doing keybytesToHex(key).

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

This can be useful when working with raw RLP data.

Issue found by @guidovranken

These functions allow destructuring of raw rlp-encoded bytes
without the overhead of reflection or copying.