Our AbstractCursor iterator helper methods return `null` to indicate that the cursor has been completely iterated.  However, using `null` to indicate the "lack of another value" is problematic in the case where a user may want to map values to `null` because after mapping a value to `null`, we perceive the cursor as being exhausted and close it.

See NODE-4609 as a recent bug report about this behavior.

If our internal `next` helpers returned documents that conformed to the `iterator` protocol (documentation here) we could allow users to map to `null` values.

Acceptance Criteria

• Refactor the internal `next` method to return an iterator object instead of the raw document result.
• Update our `Symbol.asyncIterator` implementation to call next directly and use the `done` property to determine when the cursor is exhausted instead of relying on the cursor returning a `null` value
• Refactor `cursor.next`, `cursor.tryNext` and `cursor.hasNext` to unwrap the iterator object returned from `next` and return the value