[ASTextNode] Optimize handling of constrained size to almost never recreate NSLayoutManager

This also fixes two fairly subtle but serious bugs, #1076 and #1046.

AsyncDisplayKit/TextKit/ASTextKitContext.h

@@ -30,6 +30,8 @@
                          constrainedSize:(CGSize)constrainedSize
                     layoutManagerFactory:(NSLayoutManager*(*)(void))layoutManagerFactory;
 
+@property (nonatomic, assign, readwrite) CGSize constrainedSize;
+
 /**
  All operations on TextKit values MUST occur within this locked context.  Simultaneous access (even non-mutative) to
  TextKit components may cause crashes.