OAK-1768: DocumentNodeBuilder.setChildNode() runs OOM with large tree

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1627470 13f79535-47bb-0310-9956-ffa450edef68
OAK-2132: BackgroundWriteTest uses relative paths

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1627473 13f79535-47bb-0310-9956-ffa450edef68