Implement -reclaimMemory API and switch to manually controlled content clearing.

ASDisplayNode and several subclasses had previously cleared memory-heavy
objects like the backing store and text layout manager when the node's
view
or layer is removed from a visible heirarchy.  This works great in any
system
that uses a "working range", where exiting the range removes the node
from
the hierarchy and reclaiming memory at that time is important.
However, for
standard UIViewController patterns (unused in Paper), this behavior
causes
highly undesirable thrashing (leading to visible flashes & wasteful
re-rendering of content).

After this change, node subclasses should implement -reclaimMemory if
they
need to perform any other cleanup besides backing store destruction
when they
leave a working range or other scenario where memory reduction is
valuable.

To trigger this behavior, calling code should use
-recursivelyReclaimMemory.

r=nadi

AsyncDisplayKit/ASDisplayNode+Subclasses.h

@@ -179,6 +179,7 @@
  * @discussion Subclasses should override this if they don't want their contentsScale changed.
  *
  * @note This changes an internal property.
+ * -setNeedsDisplay is also available to trigger display without changing contentsScaleForDisplay.
  * @see contentsScaleForDisplay
  */
 - (void)setNeedsDisplayAtScale:(CGFloat)contentsScale;
@@ -271,6 +272,11 @@
 // Called after the view is removed from the window.
 - (void)didExitHierarchy;
 
+// Called by -recursivelyReclaimMemory.  Provides an opportunity to clear backing store and other memory-intensive intermediates,
+// such as text layout managers or downloaded content that can be written to a disk cache.
+// Base class implements self.contents = nil, clearing any backing store, for asynchronous regeneration when needed.
+- (void)reclaimMemory;
+
 
 /** @name Description */