DataStreamPeer: Use UTF-8 QByteArrays instead of QString for message headers

DataStreamPeer: Use UTF-8 QByteArrays instead of QString for message headers

This affects the objectName in SyncMessage, InitRequest and InitData.
While the savings should be rather small in particular with compression
enabled, we shouldn't skip this opportunity; while introspecting all
serialized data for QStrings would be too invasive, we can easily do
it for those clearly defined message fields.