Reduce chunkSize to avoid internal error

[rodarima]

See #170

[insomniacslk]

Thanks @rodarima ! The large number for user pagination was done to speed up joining large Slack teams. I suspect that decreasing to 30 will defeat that speed gain, we should verify that.
Furthermore the Slack API of users.info seems to have changed compared to the slack-go implementation - the latter uses users as parameter, while (now?) the API method expects a singular user. I wonder how does this even work with a different argument

[rodarima]

Thanks @rodarima ! The large number for user pagination was done to speed up joining large Slack teams. I suspect that decreasing to 30 will defeat that speed gain, we should verify that.

I tested with a `chunkSize` of 40 users and it was already returning the internal error, so I don't thing there is much room for improvement with this endpoint. Maybe we could use the `list.users` to fetch all the users at the beginning and then ensure that the list is kept updated when joining channels (or when a user id is found to be new) by fetching only the new users (either one by one or using the legacy method).

Furthermore the Slack API of `users.info` seems to have changed compared to the slack-go implementation - the latter uses `users` as parameter, while (now?) the API method expects a singular `user`. I wonder how does this even work with a different argument

I guess they have changed the API and the documentation, but they have left the legacy working for some time. It is likely to be broken soon.