allow cancelling a resumable upload

[benjaminp]

Thanks for stopping by to let us know something could be better!

Is your feature request related to a problem? Please describe.
The underlying resumable upload API allows canceling an upload. However, the WriteChannel returned by the Storage.writer methods has no way to cancel the upload.

Describe the solution you'd like
Add an API like cancel to the object returned from Storage.write.

Describe alternatives you've considered
n/a

Additional context
Any other context or screenshots about the feature request.

[BenWhitehead]

This is a shortcoming we are aware of, but haven't yet implemented it (we're having to tease apart some things due to the fact WriteChannel us used by more than just GCS).

Some good news, resumable uploads will be automatically reaped by GCS one week after they are created if they are not finalized. In order to not finalize, simply never call writer#close()

Unfortunately I don't have an estimated of when we will be able to provide a cancel or abort for a resumable upload.