RetainShards could delete newly created shards

[guilload]

1. on startup, the cp issues retain shards request for shards 1,2,3 on ingester A in a fire and forget fashion
2. next, the cp processes a create shards request for ingester A and creates shard 4
3. ingester A receives create shards response and initializes shard 4
4. ingester A receives retain shards request and deletes shard 4