Let WriteFile::container_path be canonical container path

Previously, there are two places using container_path in WriteFile,
which might cause bifurcation.
(1) used by addPrepareWriter as canonical container path.
(2) used to create index as its physical_path, which might be canonical
    path or shadow path.

As Index::physical_path is only used in mlog, so now don't change
container_path after it has been initialized to canonical path.