This can be called by another thread.
It sets a new iterator, that will be picked up on the next reset.
iterator - the new iterator to update to.
public void reset()
When reset is called, then if a new iterator has been provided by another thread via update method,
then we switch over to using the new iterator.
It is important that on nulling off the changedIterator, we atomically compare and set as the
changedIterator could be further updated by another thread whilst we are resetting,
the subsequent update simply would be picked up on the next reset.