OAK-1941 - improve test to check larger append operations (wip)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1633581 13f79535-47bb-0310-9956-ffa450edef68
OAK-1941 - add test that verifies that parallel calls to update() really work the way they ought to

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1633591 13f79535-47bb-0310-9956-ffa450edef68