Emit download begin/finished state event for docker images

[jippi]

More than once a day I'm puzzled on why some allocations take forever to get out of "pending", and there is nothing in the state history of a task than "Received"

I'm guessing that its downloading a docker image, but no way to actually see what nomad is waiting on... amping on the detail level in the start phase would greatly improve confidence in nomad is working, and not just stuck doing something random

[schmichael]

There will be a "downloading" event emitted in nomad 0.5.3!

There's actually one emitted in 0.5.2, but a bug meant it wasn't visible until after the download finished.

Fixed in #2165 and originally added in #2054

[jippi]

Thats so baws!

[github-actions]

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