parsed value unwrapping should use locks

Although we don't like values to be used concurrently, there's a couple of places where this happens (unnest being one).
We have a mutex which is used for concurrent Field() and Index() access.
Snag is, unwrap clears fields and elements without mutex which could potentially lead to concurrent map reads and writes.
We have no such case in query, but one suspected case in the projector, so it's probably best to add concurrency control in unwrap().