SLING-3744 - optionally schedule execution of health checks via cron-like service property. Contributed by Georg Henzler, thanks!

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1614655 13f79535-47bb-0310-9956-ffa450edef68
SLING-3744 - add missing dependency - integration tests failed

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1614656 13f79535-47bb-0310-9956-ffa450edef68
SLING-3744 - increase API package version - HealthCheck has a new constant

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1614657 13f79535-47bb-0310-9956-ffa450edef68
SLING-3744 - if a health check takes a long time to execute we should wait until it's finished before executing it again

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1614659 13f79535-47bb-0310-9956-ffa450edef68