This fixes 2 issues:
- Because neoctl connects to admin nodes without requesting identification,
  the protocol version was not checked, which could even be dangerous
  (think of a user asking for information, but the packet sent by neoctl
  could be decoded as a packet to alter data, like Truncate).
- In case of mismatched protocol version, the error was not loggued on the
  node that initiated the connection.

Compatibility is handled as follows:
- For an old node receiving data from a new node, the 2 high order bytes of the
  packet id, which is always 0 for the first packet, is decoded as the packet
  code. Packet 0 has never existed, which results in PacketMalformedError.
- For a new node receiving data from an old node, the id of the first packet,
  which is always 0, is decoded as the version, which results in a version
  mismatch error.

This new protocol also guarantees that there's no conflict with SSL.

For simplification, the packet length does not count the header anymore.