huge accessor refactoring

huge accessor refactoring

Turn <'a> into <T: Clone + ReadAt>. We can still use `&'a
dyn ReadAt` in its place, but we can now also support using
an `Arc<File>` to create an Accessor of a 'static lifetime
for more convenient storage and reuse. This way the Accessor
can be thrown away after calling `open_root`.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>