Breaking the TypeTransporter::createStream function in 2.4

I'd like to inform transport developers that the
TypeTransporter::createStream function will need to return a
ChannelElementBase::shared_ptr instead of a ChannelElementBase*. This
is necessary in order to fix the refcounting of connection elements.
This change will be merged on master one of these days.

Afaikt, all known transports are affected.

Peter