OAK-1579: ConcurrentAddNodesClusterIT.addNodes2() fails on travis

Synchronize runBackgroundOperations() method to avoid concurrent calls from dispose() and background thread.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1582316 13f79535-47bb-0310-9956-ffa450edef68