Using multiple threads to consume rabbitMQ messages.

```sh
curl -XPUT 'localhost:9200/_river/my_river/_meta' -d '{
    "type" : "rabbitmq",
    "rabbitmq" : {
        "num_prefetch" : 0,
        "num_consumers" : 1,
    },
    "index" : {
        "flush_interval" : "5s",
    }
}'
```
We also removed `ordered` and `bulk_timeout` options as we don't need
them anymore.
Closes #11.