Unfortunatelly QDataStream has no way of waiting for complete data to be
available in the underlaying QIODevice, so we need to serialize the command
in two levels: we serialize the entire command into a QByteArray and then
we write the size of this QByteArray, followed by the data into the actual
socket. By sending the size of the serialized data first we allow the deserialization
code to ensure it reads the entire data (and waits for more if they are not
available in the QIODevice at once).

This fixes sending very large commands as well as sequences of commands at
some cost to performance, but that's something we can look into optimizing
later on.