SLING-2914 : adding a load test to discovery.impl - takes 140sec to run, so including in normal tests so far

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1490301 13f79535-47bb-0310-9956-ffa450edef68
SLING-2914 : fix NPE unveiled by SLING-2914 : a voting handler might be tempted to participate in a voting while the local instance doesnt send heartbeats

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1490303 13f79535-47bb-0310-9956-ffa450edef68