Refactor Message class

The `Message` class' origins date very far back and the resulting code is not overly clean. There is a lot of overlapping API surface, which can be difficult to use and understand the consequences of.

Worse, we cannot inject anything into newly created Message objects, which means that any additional behavior either needs to be passed in as extra parameters to its various methods, or has to be added to the class itself which really is not be best place to have.

The idea is to create a `MessageProvider` that can be injected wherever new `Message` objects are required. The provider can then make sure that message objects are "injected" with the necessary references to other systems (such as stats recording, message weighers, etc). By creating a special provider we can work around the overhead of injecting directly.