* Don't lock while calling downloader
Addresses #2785
To avoid performance issues, we should avoid locking the downloader.
To achieve this we need to do some kinda gross things. Essentially
the cost is the code is more complex and potentially far less performant
in edge cases. In testing, edge cases are nearly never hit, but I'm not
sure how good I feel about the cost in code complexity. This exacerbates
the locking issues in ASNetworkImageNode:
1. There is no convention for which methods lock.
2. There's no indication which vars are only set on init and therefore
safe to access except in the class extension definition.
* Shouldn't have checked in product changes.
* Using ivar instead of local var copied within lock.
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