SLING-3278 : Provide a HealthCheckExecutor service. Applied modified patch from Georg Henzler. I've changed the methods names and signatures of the executor service and did some implementation changes and code clean ups. This is not the final version, the commit should rather help going forward

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1553136 13f79535-47bb-0310-9956-ffa450edef68