Follow-up from "Use unique pointers for EventContexts": convert scheduler interface to use unique pointers only

The following discussion from !732 (merged) should be addressed:

• @clemenci started a discussion: (+2 comments)

  This worries me a bit... you should move the unique_ptr into m_finishedEvents, and popFinishedEvent should be refactored to return unique_ptr (at the moment it returns a StatusCode which value is equivalent to check if the returned pointer is valid or not, and used only in a print out... and basically ignored).

  It may also be that tbb::concurrent_bounded_queue cannot hold unique_ptrs (but I doubt it) in which case you can forget about my comment.

assigned to @bwynne

mentioned in issue #37 (closed)

mentioned in merge request !781 (merged)

Just as a recap for clarity, the suggestion is to use unique pointers for EventContexts throughout the scheduler interface. This should be possible, but is a large-scale change since it affects Athena as well.