CS-D: Refactor for greater flexibility

At the moment, the Drools executable model is created as the stream is created. When one stream turns into another, the executable model for the previous stream is set in stone. This does not allow for flexibility - in order to implement PLANNER-2003, we need to be able to make decisions about the executable model after all of its constituent parts are known.

We will refactor CS-D in the following way:

• Exec model will only be built in the scoring stream.
• No API will change, neither will any tests.
• All existing tests will pass.
• There will be no performance regression as verified by score director benchmark.