Refactor the MQTT receiver to dynamically load decoding functions based upon topic patterns

At present we have a different container for each source of MQTT messages. We can reduce this to a single container if it can dynamically load decoding functions based upon topic name patterns or similar.

Issue #28 is a prerequisite for this.

Start by reviewing prerequisite issue #28 and the current MQTT receiver and container arrangement. Map how topic names could select decoding functions; done means the separate source containers are consolidated into one container that dynamically loads the appropriate decoder based on topic patterns.

Written by the indexing model from the issue text.