static-file: properly set/use cache header for static file (closes #2255013) This changesets enables the standard http cache mechanism where the static controller may reply "304 Not modified" based on `last-modified` in HTTP response and `if-modified-since` in HTTP query. The last modified time is computed using the file-system information. The pre-existing logic using an `Expires` header to prevent client from sending request stay in place. The new logic just prevents sending the file again if not necessary.

[repository] move task manager instantiation outside repository. Repository callers are in charge of providing a task manager. The task manager is optional when the repository is not a server. This part is not really expected to work well yet. But we aim to remove all task management responsibilities from the Repository object.

[repository] split repo initialization from starting looping task (closes #2204047) This separation highlights that two distinct operations are done. This is a step towards a full distinction between repo, server and looping tasks

[repo looping task] move looping task logic in a dedicated object (progress #2204047)