Make autosubscribe a deprecated alias for autorun.

XXXXXXX needs tests.  apparently there were no tests for autosubscribe?

Replace the use of client-side sub de-duping (which mostly existed for the sake
of autosubscribe, and causes issues with server-driven unsubscribes) with making
Meteor.subscribe explicitly aware of reactivity. Other than this specific case
of reactivity, subscriptions are no longer de-duped in the client.

Expose an "invalidated" flag on Meteor.deps.Context.

Guarantee that invalidation callbacks from different contexts will not be
interleaved at flush time.