Summary: public
I had previously assumed (based on past experience and common wisdom) that `[UIImage imageWithData:]` was safe to call concurrently and/or off the main thread, but it seems that may not be the case (see https://github.com/AFNetworking/AFNetworking/pull/2815).
This diff replaces `[UIImage imageWithData:]` with ImageIO-based decoding wherever possible, and ensures that it is called on the main thread wherever that's not possible/convenient.
I've also serialized access to the `NSURLCache` inside `RCTImageLoader`, which was causing a separate-but-similar crash when loading images.
Reviewed By: fkgozali
Differential Revision: D2678369
fb-gh-sync-id: 74d033dafcf6c412556e4c96f5ac5d3432298b18
0fe074acbd