Improvements to thread safety during closely spaced edit events.

This applies primarily when beginUpdates / endUpdates is not used.
Due to user interaction-driven edits, like reloads or adding content
at the bottom, sometimes this is unavoidable in app design and thus
critical.

I have a diff in flight to make ASDataController / ASRangeController
robust against very aggressive thrash testing, which will be added
both to the unit test suite and this new example project.

examples/ASTableViewStressTest/Sample/ViewController.h

@@ -0,0 +1,16 @@
+/* This file provided by Facebook is for non-commercial testing and evaluation
+ * purposes only.  Facebook reserves all rights not expressly granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#import <UIKit/UIKit.h>
+
+@interface ViewController : UIViewController
+
+@end