Summary:

Upstreaming a fix by @ntre that fixes a crash we saw internally related to [_queue cancelAllOperations].

Calling [_queue cancelAllOperations] will release all references to any active operations.
If the blocks of those operations have a reference to itself, it will result in dangling pointers, which could conceptually trigger a later crash if there's a race between the operation completing and it being pulled out of the queue.

Add explicit strong reference while block is running.
For good measure, fix same pattern also in RCTFileRequestHandler.

Note: separately, that this code is passing the op itself as a requestToken to [delegate URLRequest:] methods is suspect. That delegate can retain said token.

Test Plan:

Tested internally.