Introduce the DataStreamProtocol

Introduce the DataStreamProtocol

This is going to be a streamlined version of the LegacyProtocol, still
using QDataStream serialization, but with some of the overhead removed
from the message formats and not supporting stuff that's deprecated.
The DataStreamProtocol will be automatically selected if both client
and core support it.

This commit simply adds a copy of the LegacyProtocol with the necessary
changes to make it compile. The optimizations are going to be in separate
commits so that the changes can be easier tracked by others implementing
it.