I had intended for this to be a small change.
I was going to just add context.Context to the BlobReceiver interface,
but then I saw blob.Fetcher could also use one, so I decided to do two
in one CL.
And then it got a bit infectious and ended up touching everything.
I ended up doing SubFetch in the process by necessity.
At a certain point I finally started using context.TODO() in a few
spots, but not too many. But removing context.TODO() will come in the
future. There are more blob storage interfaces lacking context, too,
like RemoveBlobs.
Updates #733
Change-Id: Idf273180b3f8e397ac5929c6d7f520ccc5cdce08
mpl