Enable a higher keep-alive setting

[data-sync-user]

Like syncstorage-rs does, Contile could set a higher CONTILE_ACTIX_KEEP_ALIVE value. This would be more efficient for nginx in that it wouldn’t need to setup/teardown a connection for every request. Explanation: https://www.nginx.com/blog/avoiding-top-10-nginx-configuration-mistakes/#no-keepalives

There’s a closed draft PR of this here:

https://github.com/mozilla-services/cloudops-infra/pull/4496

but it requires additional configuration on the nginx side.

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