The server prevents a client from uploading objects in a branch that another client is uploading to.
However, if the client crashes and never delete the entry it won't be possible to upload again.

Currently we upload a single object at a time but the HTTP connection overhead could be too much.
Besides we already hit a problem with "too many open files" and had to limit the number of concurrent transfers and delay each one.

Add an alternative transfer method that sends all the objects at once.