zbigMapReduce.js fails occasionally because more than 5 migrations manage finish since the beginning of either of the two bulk writes it executes, causing the test to fail since the write never establishes a shard version. Similarly to sharding_balance4.js as of SERVER-28697, we should ignore a certain number of NoProgressMade errors to make the test fail less frequently.

sharding_balance4.js and bulk_shard_insert.js occasionally fail because more than 10 migrations complete during the course of a find command exhausting mongos's retry attempts and failing the test. Modifying the test to retry a couple times on StaleShardVersion should make it fail less often.

We can also consider making a generic override for read commands that retry on StaleShardVersion errors, so it can be load-ed into tests that involve frequent migrations.