Support queries over different datasets in Odinson's API

[hillelt]

As developers of an application on top of Odinson it would be helpful to us if the dataset/index we query against could be determined dynamically, at query time.

One way to support this in Odinson is to add an optional "dataset name" parameter in the query API, and to require the user to specify the index path for each dataset in the config file before starting the API.

We think we might also get a similar effect by indexing multiple datasets into the same index and use the "parent query" feature in the API to restrict the results to a specific dataset. We haven't tried this direction though, and we're not sure if that's the intended use.