Make async index more resilient on documentmk

The async index update suffers in an eventual consistency store from stale reads of the root node. It may happen that in a cluster an indexing job running concurrently on more nodes, would not see the lease updates coming from another cluster node, so it would end up wiping away checkpoints and/or triggering a full reindex.
There is not much we can do at this level, but make the code a bit more resilient for this specific case (reindex due to missing reference checkpoint) by issuing a write+read operation to force a root revision update which would hopefully prevent a full reindex.
For background, the preferred solution is to have an upper layer choose the node where the async jobs run (aka. use sling discovery bundles).