Don't allow create if the event is already in the model store.

When creating and updating just after 'created', like this:

```js
'{Transaction} created':function(Transaction, ev, txn){

	// Update the balance.
	this.updateBalance(txn);

	// Save the model
	txn.save();
},
```

the create event can still run twice on the computer on which the txn was created, resulting in duplicate copies.

This PR checks the model store before creating to avoid possible duplicates that the _locks don't catch.