Summary:
Under rare and as-yet-to-be determined circumstances, images can sometimes fail to load/download and get "stuck", without producing an error.
Because the `RCTNetworkTask` for these images is stuck in the "in progress" state, they clog up the RCTImageLoader task queue, which has a limit of 4 concurrent in-progress tasks.
This was previously masked by the fact that we automatically cancelled image requests when the RCTImageView moved offscreen, but we no longer do that.
This diff adds logic to detect some types of stuck task and remove them, thereby unblocking the queue. I've also restored the functionality of cancelling downloads for offscreen images (but not unloading the image itself) so that stuck images will be cancelled when you move to another screen, instead of using up space in the queue forever.
Reviewed By: fkgozali
Differential Revision: D3398105
fbshipit-source-id: 75ee40d06a872ae8e1cb57f02f9cad57c459143c
235749ba19