task-list can hang if the 'published' field is big enough

'mistral taks-list' can hang if any of the 'longtext' fields contains relatively big data.

In my particular case the size of the published field contained ~3MB sized json data on average, and even with the default limit (100 entries), this can hang and possibly cause timeouts.

It is unnecessary to query all of the fields, instead the client should specify the fields that are about to be displayed. I verified the case with these queries:

curl http://localhost:8989/v2/tasks?fields=id
curl http://localhost:8989/v2/tasks?fields=published

The first one was nearly instant, while the latter took minutes to complete.