Oops, make the ID invalidation optimization actually work.

Because we normalize "id" queries to {_id: "id"}, make sure we process those
correctly.

This adds a helper function to minimongo that's only used by mongo-livedata, but
I suspect it will be useful there too.  (In fact! All of the calls to
_selectorIsId in minimongo are essentially unused, because
Meteor.Collection.update/remove normalizes first...)