Add debug logs for why we sync a file

Makes it easier to troubleshoot syncing issues.

When running in debug you'll get logs like this from the comparator module:

```
DEBUG - syncing: /private/tmp/sync/file-0 -> bucket-name/file-0, file does not exist at destination
DEBUG - syncing: (None) -> bucket-name/file-2 (remove), file does not exist at source (/private/tmp/sync/file-2) and delete mode enabled
DEBUG - syncing: /private/tmp/sync/file-4 -> bucket-name/file-4, size_changed: False, last_modified_time_changed: True
DEBUG - syncing: /private/tmp/sync/file-8 -> bucket-name/file-8, size_changed: False, last_modified_time_changed: True
DEBUG - syncing: /private/tmp/sync/file-99 -> bucket-name/file-99, file does not exist at destination
```

I was careful not to change any of the existing logic, so this should have no functional changes.