DataStreamPeer: Optimize the InitData message

DataStreamPeer: Optimize the InitData message

Since an InitData message consists of the usual message header
(serialized as a QVariantList) followed by a QVariantMap, we can
do what we did in a few other places already; transform the map
into a list of key-value pairs, the keys being UTF-8 strings (as
QByteArrays) instead of QStrings, and append that list to the
rest of the message.

This is just a small optimization, but straightforward to do without
changing protocol semantics or introspecting payloads. And
since we send lots of InitData messages, saving even a few bytes
per message may be worth it.