Currently we set parallel_image_downloads to False, which means that
all downloads that go through the image cache are serialized.
This change enables it by default and deprecates in favour of a new
more fine-grained mechanism: the new option image_download_concurrency
specifies how many downloads (and raw conversions) will run in parallel.
Update logging to trace how long each download takes.
Change-Id: I8b85afda295029f85e82143cf7d4bcb2316860f6
a9d82bb12b