parallel creation of hashed shard key for multiple collections may not distribute them evenly

This is captured by SERVER-9258 and SERVER-9260 where parallel unit tests were causing hashed key sharded collection not to be distributed evenly.

The problem is that it's a real life scenario so our test should include trying to do it in parallel and we should fix the distribution of initial chunks - when a migration of empty chunk errors, it should possibly try again several times rather than just continuing on as that leaves all initial chunks on the original shard.