Create a resource pool

[arnecls]

There are a lot of resources that could be shared between producers and/or even consumers.
E.g. if there are 3 kafka based plugins, we have 3 connections to each broker while one would be sufficient. Same goes for any other resource like files, network sockets, etc.

Implement and design a concept where resources of any kind can be added and retrieved from a central resource registry.

• It must be threadsafe
• Cleanup of used resources must be possible across plugins (e.g. via reference counting)
• We don't need a "general resource" type, as the type of the resource is defined by the plugin.

As for the last point: we could try to create something like a general resource by trying to find a general interface, but this is overkill and will create more problem than it solves. The registry should be treated like a cache, not as a hub to enable writing a generic producer or consumer.