Remove +30s to default download timeout

The 30s were added with the explanation: "Wait an extra thirty seconds to account for the call create call's [sic] potential overhead." But the timeout is only applied to http.Client.Timeout on the actual fetching of the artifact. http.Client.Timeout documentation states:

```
  // Timeout specifies a time limit for requests made by this
  // Client. The timeout includes connection time, any
  // redirects, and reading the response body.
```

Create overhead, either prior to or after the actual fetching, is independent of the Client.Timeout.

This commit also makes this singularity of purpose more explicit by consistently naming the parameter downloadTimeout (sometimes née timeout) throughout the code.

— – -

@jmyounker — I guess this was an oversight from adding the same +30s to the harpoonctl agent create/run flags? (It makes sense there.) Of course, it's certainly possible that I've overlooked something — please correct me if I have. Otherwise, your :+1: will allow me to merge this into your async-artifact-download branch, and then, from there, to master. Let me know.