Fix RCTImageLoader multi thread crash (facebook#22746)

Summary:
Fix crash similar to facebook#22410
react-native: 0.51.0
react: 16.0.0

Changelog:
----------

[iOS] [Changed] - Use onw serial queue to execute invalidate and send request action in RCTHTTPRequestHandler.mm.

Message:
--------

```
- (void)invalidate
{
  [_session invalidateAndCancel];
  _session = nil;
}

- (NSURLSessionDataTask *)sendRequest:(NSURLRequest *)request
                         withDelegate:(id<RCTURLRequestDelegate>)delegate
{
  // Lazy setup
  if (!_session && [self isValid]) {
    NSOperationQueue *callbackQueue = [NSOperationQueue new];
    callbackQueue.maxConcurrentOperationCount = 1;
    callbackQueue.underlyingQueue = [[_bridge networking] methodQueue];
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    [configuration setHTTPShouldSetCookies:YES];
    [configuration setHTTPCookieAcceptPolicy:NSHTTPCookieAcceptPolicyAlways];
    [configuration setHTTPCookieStorage:[NSHTTPCookieStorage sharedHTTPCookieStorage]];
    _session = [NSURLSession sessionWithConfiguration:configuration
                                             delegate:self
                                        delegateQueue:callbackQueue];

    std::lock_guard<std::mutex> lock(_mutex);
    _delegates = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory
                                           valueOptions:NSPointerFunctionsStrongMemory
                                               capacity:0];
  }

  NSURLSessionDataTask *task = [_session dataTaskWithRequest:request];
  {
    std::lock_guard<std::mutex> lock(_mutex);
    [_delegates setObject:delegate forKey:task];
  }
  [task resume];
  return task;
}

```

now the invalidate function is called by the RCTBridge.invalidate->RCTCxxBridge.invalidate->[moduleData.instance invalidate] , this is on the "com.facebook.react.HTTPRequestHandlerQueue".
the sendRequest:withDelegate function is called by RCTImageLoader and is on the  "com.facebook.react.imageLoaderURLRequestQueue".

when one thread step in invalidate and execute [_session invalidateAndCancel] and the another thread step in sendRequest:withDelegate and execute [_session dataTaskWithRequest:request], the _session is invalidate, so there will be a crash "Task created in a session that has been invalidated"
Pull Request resolved: facebook#22746

Differential Revision: D13781512

Pulled By: cpojer

fbshipit-source-id: bd5fd1edf593e2bcdcc18596a29e906882bac8a4

Libraries/Network/RCTHTTPRequestHandler.mm

@@ -23,13 +23,16 @@ @implementation RCTHTTPRequestHandler
 }
 
 @synthesize bridge = _bridge;
+@synthesize methodQueue = _methodQueue;
 
 RCT_EXPORT_MODULE()
 
 - (void)invalidate
 {
-  [_session invalidateAndCancel];
-  _session = nil;
+  dispatch_async(self->_methodQueue, ^{
+    [self->_session invalidateAndCancel];
+    self->_session = nil;
+  });
 }
 
 - (BOOL)isValid
@@ -73,8 +76,10 @@ - (NSURLSessionDataTask *)sendRequest:(NSURLRequest *)request
                                            valueOptions:NSPointerFunctionsStrongMemory
                                                capacity:0];
   }
-
-  NSURLSessionDataTask *task = [_session dataTaskWithRequest:request];
+  __block NSURLSessionDataTask *task = nil;
+  dispatch_sync(self->_methodQueue, ^{
+    task = [self->_session dataTaskWithRequest:request];
+  });
   {
     std::lock_guard<std::mutex> lock(_mutex);
     [_delegates setObject:delegate forKey:task];