This is a more general case of the problem encountered in SERVER-20666. The attached fair_balancer_round.log also demonstrates this problem on another test.
fair_balancer_round.js fails intermittently (movePrimary returns error "database foo not found")
Metadata of second mongos is out of date in jstests/sharding/shard3.js
coll_epoch_test1.js needs to wait until movePrimary is committed on the config servers