[ASMultiplexImageNode] Avoid holding on to its lock while calling out to downloader

[nguyenhuy]

[appleguy]

@nguyenhuy There's another case of the same pattern just below, in -didEnterVisibleState. Did you happen to see or by code inspection identify any risk of lock interleaving?

We should decide whether the node's lock or the downloader's lock is higher precedence, and then as long as we are using them in that sequence, it would not be necessary to unlock when calling the other.

[nguyenhuy]

I just happened to look at this method and noticed that we're doing it differently in ASNetworkImageNode.

[CLAassistant]

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