Phase out `transport_drivers`

[mojomex]

While transport_drivers was a good and convenient starting point for network communication in Nebula, requirements to error handling and asynchronicity, as well as linking issues (ALL of Boost is linked to basically all of Nebula) with the Boost version of transport_drivers currently used, have made it a less attractive choice going forward.

Features like #231, #235 aim at replacing transport_drivers step-by-step.

This issue is considered done when the dependency on transport_drivers is removed from Nebula (all vendors) completely. The use of Boost.ASIO and Boost.Beast is fine going forward.