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Refactor the recursive filesize scanner

- Now caches filesize for all files and directories
- Moves locking into the caller so it can be accessed prior to shutdown
- `::get_site_size()` now correctly accounts for excluded files
- Extend `::filesize()` with the ability to skip excluded files when calculating a directories total size
- Use `::filesize()` in `::get_site_size` instead of duplicating logic
Merge branch 'master' of github.com:humanmade/backupwordpress