Provide a copy of the event to actions

[jsccast]

Since an action (e.g., a Javascript action) can actually modify the event and since in general action executions for a single event could occur concurrently, then action should receive a copy of the event.

(It's a little strange to modify the event object itself in a rule action, but that approach might be convenient for, say, forwarding the event with some modification.)

[jsccast]

Turns out to be non-trivial because the binding for ?event is injected early in the rule evaluation.