(timebox) Handle collection duplicates

Drive sync can't handle two documents with the same uid. This is not possible with regular folders and workspaces but a document can appear as many times as there are collections that include it.

If a document c.txt is created in the folder A and added in the collection B, and that A and B are both synchronisation roots, Drive will see a duplicate uid and the following error will be in the logs:

 WARNING  nxdrive.engine.watcher.remote_watcher Illegal state: a remote creation cannot happen if there already is the same remote ref in the database

So c.txt will only be present in A, and Drive won't show any error to the user.

The best solution would be to create a symlink in B pointing to A in case this happens, and replacing it with the file if A is unsynchronised by the user.