a8n: Increase default fetch timeout from 2s to 10s

Created by: mrnugget

Relevant Slack thread: https://sourcegraph.slack.com/archives/CMMTWQQ49/p1575934344362800

@christinaforney reported 503 errors when previewing a new campaign. We suspect that this is most probably due to replacer's cache being empty and fetching the ZIPs from gitserver taking more than 2s.

2s was already pretty optimistic. So let's try 10s which should give us a lot more headroom without being problematic.