OAK-1246 - Make AsynchIndexUpdate task to run only on a single node in a cluster

Implemented the support by making use of
1. async-status flag along with timeout handling
2. Also restricting the scheduler to run job on only one node. Default scheduler
   does not support it. However say when running in Sling this would be used

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1547289 13f79535-47bb-0310-9956-ffa450edef68