hg-inner-http: Increase of Gunicorn graceful timeout
One of our users got `abort: error: EOF occurred in violation of protocol (_ssl.c:1123)` while cloning over HTTPS (from a CI job of another project, but it should not matter). It turns out that there was a Gunicorn worker restart at about the same time. The Gunicorn `--graceful-timeout` option ([doc](https://docs.gunicorn.org/en/20.1.0/settings.html#graceful-timeout) is probably the relevant ones, and according to various documentation clarification issues on GitHub, eg [this one](https://github.com/benoitc/gunicorn/issues/3129), it seems to be independent from `--timeout`. Perhaps it is time to do #13 in the same move to give our users the way to tweak it themselves.
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