Refactor to add ABANDONED message

[mdickinson]

This is a slight cleanup of the BaseFuture and wrapper logic.

Previously, when execution of the background task's callable was abandoned due to cancellation, we acted as though it had run anyway, sending a STARTED message followed by a RETURNED message (with an argument of None) to the future. That was slightly smelly (inventing a return result when none existed), and involved sending and processing two messages when one would have been sufficient.

This PR introduces a new ABANDONED message for this. It also:

• expands the CANCELLED future state into three different internal states, depending on whether the task's callable completed, failed, or was never executed (abandoned)
• keeps the result and/or the exception information from the background task even when cancellation occurred. It's private, but it's there to be used during debugging if necessary
• annotates the semi-private methods in the BaseFuture to add explicit state changes

This PR is pure refactoring: it introduces no user-visible changes. The tests for the internals of the BaseFuture did need to be updated to take account of the extra message type. I'll self-merge after self-review.