MiniSolrCloudCluster doesn't use a thread-safe list for its jetties

MiniSolrCloudCluster uses a LinkedList<> to hold its jetties. However, multi-threaded startup can break this because the list isn't thread safe. We should use CopyOnWriteArrayList instead.

See for example http://jenkins.thetaphi.de/job/Lucene-Solr-trunk-Linux/14819/, which starts up 5 nodes but then only has 4 entries in its jetty list, causing assertion errors and thread leaks.