Utility class for automatically cancelling a context after an operation

CancellableContext must always be cancelled at the end of the request. Let's add a utility that simplifies the common cases, for example:
- run a Runnable, and then cancel when done or exception thrown
- run a Callable, and then cancel when done or exception thrown
- run a Callable>, and cancel when done, or exception thrown, or ListenableFuture cancelled by user