[TEST] Stress test for update and delete concurrency.

This test deletes and updates using upserts documents over several threads in a
tight loop. It counts the number of responses and verifies that the versions at
the end are correct.
[TEST] Stress test for update and delete concurrency.

This test deletes and updates using upserts documents over several threads in a
tight loop. It counts the number of responses and verifies that the versions at
the end are correct.