SLING-4048 : Avoid keeping jobs in memory. Another refactoring: move job cache to queue (WiP)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1632617 13f79535-47bb-0310-9956-ffa450edef68
SLING-4075 - added some set of simple missing unit tests

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1632621 13f79535-47bb-0310-9956-ffa450edef68