Store.gather / Store.defrag: limit them to items of the given namespace

[ThomasWaldmann]

Follow-up to #231.

Store.gather and Store.defrag now require all items (and the defrag target) to be in the given namespace: item names are given without the namespace, and a / in an item name raises ValueError. A name with a / could address an item in a deeper namespace (e.g. sub/x in namespace data belongs to data/sub/ if that is configured), with other nesting levels and another cache policy.

This simplifies Store.gather: all ranges are in one namespace, so they all have the same cache policy. gather now either reads all ranges via the cache path (writethrough / mirror namespace) or gathers all of them from the backend with one call. The per-range cache policy lookup and the merging of cached and backend ranges are gone.

• _find_sources: shared by gather and defrag - validates the sources, rejects / in item names, maps the names to nested backend names (one find per distinct name).
• The backend-level gather / defrag are unchanged (they get full backend names).
• borg is not affected: it calls defrag with namespace="packs" and plain hex names.
• Tests: new test for the namespace rule (gather, defrag sources and target); the one cache test that mixed namespaces in a single gather call is removed.
• Docs + changelog.

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