SLING-5195 : HeartbeatHandler now has a dedicated separately scheduled Runnable that periodically checksForTopologyChanges - to avoid possibility of being blocked by long-during session-saves. Additionally, it checks if its own heartbeat is older than a heartbeat timeout - in which case it treats itself as TOPOLOGY_CHANGING.
sling/trunk/bundles/extensions/discovery/impl/src/main/java/org/apache/sling/discovery/impl/DiscoveryServiceImpl.java