Fixes a deadlock caused by walking up the heirarchy. (#2147)

* Fixes a deadlock caused by walking up the heirarchy.

* Use scope locker / unlocker and add a comment.

* Add comment about calling __setNeedsLayout without the lock.

AsyncDisplayKit/ASDisplayNode.mm

@@ -1182,6 +1182,9 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
   // but automatic subnode management would require us to modify the node tree
   // in the background on a loaded node, which isn't currently supported.
   if (_pendingViewState.hasSetNeedsLayout) {
+    //Need to unlock before calling setNeedsLayout to avoid deadlocks.
+    //MutexUnlocker will re-lock at the end of scope.
+    ASDN::MutexUnlocker u(__instanceLock__);
     [self __setNeedsLayout];
   }
 
@@ -1213,6 +1216,7 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
   [self displayImmediately];
 }
 
+//Calling this with the lock held can lead to deadlocks. Always call *unlocked*
 - (void)__setNeedsLayout
 {
   ASDisplayNodeAssertThreadAffinity(self);