Add logging around getReferenceCheckpoint method

04.03.2019 06:42:46.956 WARN [sling-oak-3-org.apache.jackrabbit.oak.plugins.index.AsyncIndexUpdate-async] org.apache.jackrabbit.oak.composite.CompositeNodeStore Checkpoint f7e6524d-62c4-4589-8137-ca873cec934b doesn't exist

The above warning was observed - but there are not enough logs around this to identify what's causing this .

We need to add stack trace when this is logged and probably changed the message to make it unique.

Also , Since this was logged from a flow in AsyncIndexUpdate - It would make sense to add some more info/ warn level logging there around the code that calls getReferenceCheckpoint()