[ASDisplayNode] Create placeholders off main when possible.
[ASDisplayNode] Create placeholders off main when possible.

This is a fix for https://github.com/facebook/AsyncDisplayKit/issues/2620,
which is a followup to https://github.com/facebook/AsyncDisplayKit/pull/2610

This will ensure thread affinity is respected for .contents but still run
placeholderImage off-main when possible (node isn't loaded).

harryworld/AsyncDisplayKit,romyilano/AsyncDisplayKit,JetZou/AsyncDisplayKit,maicki/AsyncDisplayKit,romyilano/AsyncDisplayKit,rmls/AsyncDisplayKit,lappp9/AsyncDisplayKit,flovouin/AsyncDisplayKit,lappp9/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,harryworld/AsyncDisplayKit,flovouin/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,rmls/AsyncDisplayKit,rmls/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,harryworld/AsyncDisplayKit,maicki/AsyncDisplayKit,JetZou/AsyncDisplayKit,rmls/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,flovouin/AsyncDisplayKit,JetZou/AsyncDisplayKit,flovouin/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,lappp9/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,maicki/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,romyilano/AsyncDisplayKit,harryworld/AsyncDisplayKit,lappp9/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,maicki/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,flovouin/AsyncDisplayKit,maicki/AsyncDisplayKit,JetZou/AsyncDisplayKit,JetZou/AsyncDisplayKit,romyilano/AsyncDisplayKit,harryworld/AsyncDisplayKit,rmls/AsyncDisplayKit