This implementation uses a collector pattern to allow embedded ExecutionContexts to stash Issue objects for later processing by a non-embedded ancestor ExecutionContext.
Also:
- Provide explicit constructor implementations for ExecutionContext
- Leverage explicit constructors to solidify immutability of several ExecutionContext class members.
- Fixed erroneous call to ExecutionContext.Complete in CompositeActionHandler.cs
- Use a consistent timestamp for FinishTime in ExecutionContext::Complete
