a8n: Bump defaultFetchTimeout from 10s to 30s

Created by: mrnugget

After I ran into a few 503 errors returned from replacer, I decided to take a closer look.

With a cold cache (rm -r /tmp/replacer-archives/*) and running over all ~180 repositories in my Sourcegraph instance, the 10 slowest ZIP-fetch requests take between 2 and 12 seconds:

$ awk '{print $3}' < ~/work/comby_logs_180_repos.txt | sort -g | tail -n 10
2.894504976
3.152952589
3.920574042
4.536477702
4.707850896
5.56331578
5.8037550719999995
6.3973531
7.639939148
12.110905126

After bumping the timeout to 30s I haven't run into any timeouts and since I don't see a reason why we shouldn't increase it, let's use 30s.