Shard metadata never be expired

I tried automatic shard expiration with the following database configuration:

```
[
{
  "database": "test",
  "spaces": [
    {
      "name": "everything",
      "retentionPolicy": "10m",
      "shardDuration": "1m",
      "regex": "/.*/",
      "replicationFactor": 2,
      "split": 1
    }
  ]
}
]
```

Then, old shards were successfully dropped:

```
[07/18/14 11:43:36] [INFO] DATASTORE: dropping shard /home/shugo/work/influxdb/cluster_test/1/db/shard_db_v2/00003
```

However, "GET /cluster/shards" returns dropped shards.

I guess shard metadata should be removed by updateOrRemoveShard() in all nodes, or shards should be dropped through Raft.


<!---
@huboard:{"order":767.0,"milestone_order":767.0,"custom_state":""}
-->