OAK-1647 - AsyncIndexUpdateTask creating too many checkpoint

Moved the check for job already running before the checkpoint is created. This would ensure that lesser number of checkpoint get created if job is already running

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1583269 13f79535-47bb-0310-9956-ffa450edef68