Fix the transfer progression reset if it was paused at startup

Starting the app with paused transfers will reset their progression. If a transfer is then resumed, its progression will be recomputed and correct.

But the progression should be kept unaltered when it is paused.

There is also a display bug when the chunk_size used by the uploader is not the same as the one defined in the transfer database. Resulting in a 100% progression, for instance, when the real chunk_size is 5 MB but the one in database 20 MB; and the transfer is ongoing ...