Optimize / Simplify how the IsPlayed flag is updated for CompoundTasks

Follow up to: https://github.com/Azure/azure-functions-durable-js/pull/405/files#r1081924966

Our CompoundTasks have their result and `isPlayed` flag potentially set whenever their sub-tasks/children complete. When setting the `isPlayed` flag for these tasks, we're iterating over the entire set of sub-tasks, which in the extreme case may be too expensive. We should consider optimizing this logic to leverage the iterative execution style of DF.

Start with the discussion at pull request 405, especially the referenced review comment, then trace how CompoundTasks update their result and isPlayed flag when child tasks complete. Done means preserving those behaviors without iterating over the entire set of sub-tasks in the update path.

Written by the indexing model from the issue text.