fix(dataobj): Avoid deadlock in index-builder when cancelling mid-build

[benclive]

What this PR does / why we need it:
Fixing a deadlock that can occur in the index-builder if the build context is cancelled mid-build.

• I believe this occurs on line 382 (of the updated file), when the build process exits on cancelled context before reading all values from the downloadedObject channel.
• There is a skipMode mechanism which is meant to drain the channel of pending downloads if a build exits, but it didn't trigger in this case. I decided to replace it with different semantics for the downloadWorker and a defer: Every request receives a response, and the download worker only exits when it's input channel is closed.
• I wasn't able to write a test for this case: I'm open to suggestions for how to do it

[benclive]

I think this early return is the cause. Returning here will stop reading from the downloadedObjects channel, despite us writing len(events) to it in the previous for-loop.
Nothing else reads from the downloadedObjects channel, and it has a small buffer to avoid having too many objects in memory at any one time. The next build could fill the input downloadQueue with events, block, and never be able to proceed to reading from this channel.