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 is probably the relevant ones, and according to various documentation clarification issues on GitHub, eg this one, it seems to be independent from --timeout.

Perhaps it is time to do #13 (closed) in the same move to give our users the way to tweak it themselves.