Log disconnect reason when receiving disconnect packet during RLPX handshake

[halfalicious]

Running a private build of #5568 and I'm seeing some nodes which respond to our auth with an auth-ack then send us a Disconnect message instead of a Hello message. For example:

DEBUG 04-14 21:58:07 p2p  rlpx   p2p.connect.egress: HelloPacket to ##15ac307a…@52.176.7.10:30303
DEBUG 04-14 21:58:07 p2p  rlpx   p2p.connect.egress: Frame header from ##15ac307a…@52.176.7.10:30303
DEBUG 04-14 21:58:07 p2p  rlpx   p2p.connect.egress: Successfully decrypted frame header from ##15ac307a…@52.176.7.10:30303. Validating contents...
DEBUG 04-14 21:58:07 p2p  rlpx   p2p.connect.egress: Frame body from ##15ac307a…@52.176.7.10:30303
DEBUG 04-14 21:58:07 p2p  rlpx   p2p.connect.egress: Invalid packet type. Expected: HelloPacket, received: DisconnectPacket (##15ac307a…@52.176.7.10:30303)
DEBUG 04-14 21:58:07 p2p  rlpx   p2p.connect.egress: Handshake Failed (##15ac307a…@52.176.7.10:30303)

According to the Disconnect message description in the RLPX spec, the message can include an optional reason:

reason is an optional integer specifying one of a number of reasons for disconnect:
    0x00 Disconnect requested;
    0x01 TCP sub-system error;
    0x02 Breach of protocol, e.g. a malformed message, bad RLP, incorrect magic number &c.;
    0x03 Useless peer;
    0x04 Too many peers;
    0x05 Already connected;
    0x06 Incompatible P2P protocol version;
    0x07 Null node identity received - this is automatically invalid;
    0x08 Client quitting;
    0x09 Unexpected identity (i.e. a different identity to a previous connection/what a trusted peer told us).
    0x0a Identity is the same as this node (i.e. connected to itself);
    0x0b Timeout on receiving a message (i.e. nothing received since sending last ping);
    0x10 Some other reason specific to a subprotocol.

We should log this reason (if it's available in the packet).