We need to start using context.Context for handling cancellation-type behaviors within the SourceManager. There are two parts to this:

• A context.Context should be passed to NewSourceManager(), and the cancellation channel there used to replace the exposed signal handling system. (It's still fine to have a helper func that sets up a Context for this purpose, though.)
• SourceManager methods that touch disk or network - so, pretty much all of them - also need to take a context.Context, so that the caller has the option of controlling timeouts or forcing terminations.

There's no way this is feasible without a wider refactor of *SourceMgr to use channel-based brokers for all its activity, but that's fine - that absolutely needs to happen anyway. We can do all of that at the same time.

re: golang/dep#160