Make Meteor.autosubscribe a deprecated alias for Meteor.autorun.

Instead of a general client-side sub de-duping mechanism (which mostly existed
for the sake of autosubscribe, and causes issues with server-driven
unsubscribes), make Meteor.subscribe explicitly aware of reactivity.

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

Guarantee that invalidation callbacks from different contexts will not be
interleaved at flush time. This has the implication that if you do

   context1.onInvalidate(function () {
     context2.invalidate();
   });

and this is the only way to invalidate context2, then context2's invalidation
callbacks will not be called until after *ALL* of context1's callbacks are
called. This allows us to be sure that the "unsubscribe, unless autorun tried to
re-create an identical sub" logic runs after the autorun function is rerun.