Major Description
OAK-2066 introduces the bulk createOrUpdate method in the DocumentStore interface and implements it in the Mongo and RDB DSes. Both implementations try to apply the bulk changes in a few iterations. The iteration first reads the current documents and then updates them. If there's a concurrent modification between read and update, it causes a conflict. Such documents will be updated in the next iteration. The documents that can't be updated in a few trials are eventually updated in a sequential way.
Some documents are more probable to cause a fail than other. Eg. the root document is very likely to be changed by some other process. We could create a list of such "hotspot" documents and exclude them from the bulk updates. The list can be self-maintaining, eg. documents which conflicted in more than 50% cases in the last 1h.
