Duplicate calls to start cause duplicate uploads

[Acconut]

When start() is called another time while the upload is already running, it will cause two or more uploads for the same file to be running at the same time. This can lead to hard to diagnose issues like transloadit/uppy#4815, especially since the start method is used for creating and resuming an upload.

tus-js-client should keep track of its current state and prevent duplicate calls to start. This can be implement based on feedback from #355.