there is no need to re-calculate the shares after a touch

We use touch in to cases:

- Create a new empty file
- Update the mtime

In both cases it is not necessary to re-calculate the shares. In case of the mtime update, which happens most of the time when the sync client uploads a file, we calculated the shares twice. Once if the upload was finished and a second time after the touch. So this changes should give us also some performance improvements.

cc @PVince81 we discussed this issue already, maybe you can have a look? Thanks!