Buffer Improvements: refactor buffer configuration code to handle existing and chained styles

[tobz]

Part of the Buffer Improvements RFC (RFC, #9476)

We need to refactor the existing buffer configuration code so that it can suitably represent both the existing configuration style and the new chained style.

In practice, what this will likely end up being is a custom serializer implementation that can parse the current style, but when toggled to a specific mode ("overflow" being the obvious candidate here), it would parse additional fields related to the chained buffers.

One part we'll need to flesh out here is how much we can reasonably stray from the existing style. There's likely some cookie-cutter buffer topologies we could construct based purely on the existing configuration fields, but if a user opts into the advanced configuration "mode"... do we:

• make the other fields invalid and everything has to be done in the advanced mode?
• do we use the existing fields to define the first buffer, and advanced mode defines any subsequent buffers? etc

This will likely require some noodling with what is reasonable possible in a custom serializer implementation, and what ends up looking reasonable when written in a configuration file.