LUCENE-4953: Fixed ParallelCompositeReader to inform ReaderClosedListeners of its synthetic subreaders. FieldCaches keyed on the atomic childs will be purged earlier and FC insanity prevented.  In addition, ParallelCompositeReader's toString() was changed to better reflect the reader structure.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1476526 13f79535-47bb-0310-9956-ffa450edef68