C++: Introduce LogListener class

This MR supersedes !441 (closed) by following the decision to implement pools with specific functionality outside the code library. This is a port of the relevant functionality from the MR. There are a few things which likely have to change:

• Right now all methods are exposed, including the ones allowing to change subscription topics. Ideally this would happen through an active subscription or unsubscription (as it is already) without exposing these methods.
• Since this is now constrained to subscribing to log topics, probably the interface could be simplified to allow directly setting log levels plus topics, and include higher log levels automatically.

Probably best to discuss this in person once.