core: Initialise `progress.bytesTotal` as soon as upload starts
core: Initialise `progress.bytesTotal` as soon as upload starts

This way our ETA calculations etc. will be correct even if it takes a
very long time for the first progress event comes in. This helps
especially with XHRUpload's `limit` option, where queued uploads will
not be firing progress events at all until other uploads have finished.

transloadit/uppy,transloadit/uppy,transloadit/uppy,transloadit/uppy