Speedup index rebuilds but using ADD rather than UPDATE statements when safe

The DistributedExecutorMassIndexer relies on update commands to index the new entries, but when we just run a full purge of the index it should be safe to use Add commands instead if we know that there aren't concurrent updates.

The lack of concurrent updates might be tricky but I think we can find such a condition when using Affinity as the index is being fully controlled by the current indexing engine, similarly to the checks to see if it's safe to use an NRT workspace.