Upload Refactor

[peterjan]

I was seeing how to implement etags and got side-tracked on a mini-refactor. It's a bit hard to see in the diff but it does deduplicate a bunch of code we had. I went back and forth on it a bit so definitely don't mind to discuss it further.

I extended the worker with two methods:
func (w *worker) upload
func (w *worker) uploadMultiPart

that each wrap an upload and then handle the object and (potential) partial slab data with the bus. I moved this in and out of the upload manager but eventually figured that it should not be handled by the upload manager, at the cost of duplicating persisting the partial slabs and setting it on the object.

Most importantly we now spin a goroutine to persist packed slabs, so the request that pushes it over the limit isn't blocked by the upload. I think this is better and it gets rid of duplicate logic we had in regular upload vs multi-part uploads. (This is the thing you did initially and then ran into an error, so I hope the error you ran into is in a test we still have so it flags if I made the same mistake).

It also extends the uploadConfig with everything we can gather from the upload parameters and greatly simplifies the interface of the upload manager. Now we return the etag and a potential error but eventually I'll return the object.Object that has the etag.