If a file locked in one side is deleted in the other side, the locked file is undeleted in both side after sync

[halueda]

If a file locked in one side is deleted in the other side, the locked file is undeleted in both side after sync.

1. In one side a file is locked, e.g., opened by Excel application.
2. The file is deleted in the other side directory
3. sync tries to delete this side file but failed because it is locked
4. next sync found the file exist and misunderstood it is newly placed
5. The file is copied back to the other side, and it looks like undeleted.

[halueda]

The problem exists begween step 3 and 4.
It should check whether the file is really deleted, after 3.
And, if the deletion is failed, then _last.json should keep the existance of the file.
Then step 4 should not happen.