Move sitemap files from tmp dir instead of copy

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

The `mirror` method is copying all sitemap files from tmp folder. The problem with this is that read/write operations are not atomic (and `copy` method in `Filesystem` is using no locks), so the xml files can be corrupted if two processes are trying to read/write (copy) their own versions of sitemaps to the public directory.