[Bro-Dev] Broker & CAF includes

[Matthias Vallentin]

Thanks for chiming in, Jon.

> [..] i.e. treat the use of CAF an implementation detail.

This is the clean way to think about layering and creating abstractions.
It applies to the API perspective, though. As long as CAF internals are
hidden from a Broker user, we are good.

The "implementation detail" maxim lead to artifacts like PIMPL. This
certainly made sense at the time where we considered multiple messaging
backends. At this point, we are invested into CAF, and I don't think
switching will happen anytime soon. Therefore, I don't think we need to
keep up the implementation-hiding abstractions, such as PIMPL, which
come at the cost of development productivity and performance (they are
essentially a compiler firewall due to type erasure). 

Moving forward, I plan to remove the PIMPL design while keeping CAF
hidden from the Broker API, but we'll see more CAF code in Broker
headers. That's fine in my thinking, because anyone developing and
compiling a Broker application must have CAF installed anyway.

> At the time, the risk of a copied version getting outdated seemed a
> lower priority to me than keeping Broker’s interface/design more
> simple/coherent in my head.

And to be clear: that rationale totally makes sense in this context and
at the time of writing.

    Matthias