SLING-2827 : several improvements/fixes:
 * TOPOLOGY_CHANGED is now sent with a delay after TOPOLOGY_CHANGING - this is to avoid 'event floods' during topology changes
 * after a TOPOLOGY_CHANGED event is sent, a heartbeat is triggered. This speeds up change propagation. Yet it is by no means
   the case that changes now propagate as fast as possible - it is still possible for them to propage only after the
   heartbeat-interval (for downstream connector-clients)
 * you can now configure a number of topology connectors - this replaces the 'manual' way of connecting in the webconsole
   since that is more confusing than helpful: connectors are now only created via config
 * announcements sent by topology connectors now carry a version for paranoia compatibility checks
 * connector servlet now accepts duplicate/loops but marks them as such - this information is also visible in the webconsole
 * connector requests are now PUT and DELETE (rather then POST), and are done using a url of the form 'connector._sling-Id_.json'
 * logger.debug wrapped in isDebugEnabled
 * a fix with properties changing (cannot do ModifiableHashMap.iterator.remove)
 * a fix with proper closing of a resourceResolver to avoid unclosed sessions

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1476138 13f79535-47bb-0310-9956-ffa450edef68