Use `rtm.connect` method over `rtm.start` to get websocket URL.

[aoberoi]

Description

On very large Slack teams, the rtm.start payload can be too large to deliver before a timeout (server timeouts would return response 500, but also the websocket URL itself is short-lived so the timeout could be from the time it takes for the client to read the response).

The Slack platform has a new method called rtm.connect which is much smaller and faster to load. It also would provide the websocket URL.

I think this can be implemented as a swap of one for the other because the rtm.start data is not being returned to the user anyway (see #181), so that means there's no breaking API change by not having all the same data. The point about that data being valuable is valid though, so I propose that the default would be to use rtm.connect (better for most people anyway) and there is an option for using rtm.start, and in both cases the data is returned.

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• bug
• enhancement (feature request)
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