Added `changed` callback and factored out shared --> local conversion.

Meteor local mirror

//common to client and server
SharedCollection = new Meteor.Collection('shared');

//client from here on out

LocalMirror = new Meteor.Collection(null);

var convertSharedToLocal = function(sharedDoc) {
    var localDoc = LocalCollection._deepcopy(sharedDoc); // undocumented API, might change

    // create a new id for the local doc, but store the old one
    localDoc.sharedId = localDoc._id;
    delete localDoc._id;
};

// should only run this once, and before a subscription has pulled 
// the shared data from the server.  Otherwise I'm not sure that 
// the added callback will do the initial populate step.
SharedCollection.find().observe({
    added: function(doc, beforeIndex) {
        if (LocalCollection.findOne({ sharedId: doc._id })) {
            console.log('already have a local doc with that id...');
            return;
        }
        var localDoc = convertSharedToLocal(doc);
        LocalCollection.insert(localDoc);
    },
    changed: function(newDoc) {
        var localDoc = convertSharedToLocal(newDoc);
        LocalCollection.update({ sharedId: localDoc.sharedId }, localDoc);
    },
});