Hold DNS entries sorted case-insensitively until just before sending

[bboreham]

Instead of sorting when creating the outbound GossipData object, we sort in Encode().
Fixes #1603 and #1610

[rade]

needs to be branched off 1.2

[rade]

I reckon the gossip data decoding code should be moved near the encoding code. e.g.

func (g *GossipData) Decode(msg []byte) error {
    if err := gob.NewDecoder(bytes.NewReader(msg)).Decode(g); err != nil {
        return err
    }
    sort.Sort(CaseInsensitive(g.Entries))
    return nil
}

and then call it from Nameserver.receiveGossip like this:

    var gossip GossipData
    if err := gossip.Decode(msg); err != nil {
        return nil, nil, err
    }
    if delta := gossip.Timestamp - now(); delta > gossipWindow || delta < -gossipWindow {
        return nil, nil, fmt.Errorf("clock skew of %d detected", delta)
    }
        ...

(I think that works)

[rade]

the unit tests failed.

[bboreham]

Wrong branch; replaced by #1615