Request on /splits takes too long on red_caps
reported by @mariosasko (thanks)

https://huggingface.co/datasets/red_caps showed "server-side error", which generally means a timeout from the node code that fetches the data

Indeed, while not in the reverse-proxy cache, the response to http://datasets-server.huggingface.tech/splits?dataset=red_caps takes too long (a lot more than 1.5s for sure, maybe 20s--I didn't measure the time). Note that this dataset has... 1741 splits (1741 configs, each one with the `train` split)! Nevermind: it should not take that long to generate.

We have to optimize the time taken by this request, and add tests to ensure we always are able to serve the response in a short time.

Related to #4 
Request on /splits takes too long on red_caps: reported by @mariosasko (thanks)

https://huggingface.co/datasets/red_caps showed "server-side error", which generally means a timeout from the node code that fetches the data

Indeed, while not in the reverse-proxy cache, the response to http://datasets-server.huggingface.tech/splits?dataset=red_caps takes too long (a lot more than 1.5s for sure, maybe 20s--I didn't measure the time). Note that this dataset has... 1741 splits (1741 configs, each one with the `train` split)! Nevermind: it should not take that long to generate.

We have to optimize the time taken by this request, and add tests to ensure we always are able to serve the response in a short time.

Related to #4 