Fix leak of file upload objects
After an upload completed, we were failing to delete the details of the upload
from the list (because the comparison was bogus), meaning we leaked an object
each time.
While we're in the area:
- make the request methods take an opts object (instead of a localTimeout
param), and deprecate the WithPrefix versions.
- make the non-xhr path for uploadContent use authedRequest instead of
rolling its own.
- make cancelUpload use the promise.abort() hack for simplicity
