[WIP] Lock file by parentid childname

[butonic]

In order to be able to lock a new node during upload of a new file we need to be able to lock a node that does not exist, which would work by a parentid&childname based lock. buuut that has problems.

Another solution would be to create the node with the InitiateUpload call. Similar to an fopen it should always create a 0 byte file. This will generate an id that can subsequently be used. IMO we need a 0 byte file so clients can detect that a new file was created. They can then check the upload progress and even get access to the postprocessing status.

But... this needs to be compatible with the sync algorithm used by the clients.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.