IJPL 173473: fix ArrayIndexOutOfBoundsException when using ContainerUtil.concat()

[dieterstueken]

Context

The method ContainerUtil.concat(list1, list2) works fine with immutable lists. However, if the lists are modified concurrently, iteration may fail. see: IJPL-173473

This fix contains a unit test ContainerUtilTest.testConcatenatedDynamicListsAreIterable to disclose the problem.
Surprisingly it does not indicate any problem, if you start joining two empty lists. This is, because the size of the composed list is calculated in advance and never change. So, while an underlying List is growing, the composed List just does nothing. But I think this is also not the expected behavior. But if any List shrinks, the concatenated List throws an IndexOutOfBoundsException.

To fix this, I introduced an iterator() method for the concatenated List to get the actual elements from both lists.

Other utility methods here, using a straightforward new AbstractList<T>, may suffer from the same problem but are not fixed so far.