Mac RCTDisplayLink fix plus bonus deprecated API cleanup (microsoft#854)

Defensive changes to Mac RCTDisplayLink

Real-world data suggests crashes with this stack:
  CVDisplayLink::start()
  -[RCTDisplayLink updateJSDisplayLinkState] (RCTDisplayLink.m:157)
  -[RCTDisplayLink registerModuleForFrameUpdates:withModuleData:]_block_invoke (RCTDisplayLink.m:67)
  -[RCTTiming timerDidFire] (RCTTiming.mm:324)
  -[_RCTTimingProxy timerDidFire] (RCTTiming.mm:93)

Some symbols are missing in this stack, presumably due to compiler optimizations.
-updateJSDisplayLinkState is calling CVDisplayLinkStart as a result of a
call to "_jsDisplayLink.paused = NO".
-registerModuleForFrameUpdates block is presumably getting called via pauseCallback,
likely via [RCTTiming startTimers], presumably owned by RCTCxxBridge.

The most likely immediate explanation for the crash is that we are calling
CVDisplayLinkStart with a zombie _displayLink pointer.
However there is a lot of indirection here as well as thread-hopping, and
unfortunately no clearly incorrect code that would explain such a zombie pointer.

Some defensive changes:
-explicitly remove the association to pauseCallback when underlying display link object is invalidated.
-remove a prior attempt at additional check in updateJSDisplayLinkState itself as it is not relevant.
-make sure we explicitly set _displayLink to NULL when we release it, such that there is one less failure point.

Author: ntre

React/Base/RCTDisplayLink.m

@@ -95,6 +95,15 @@ - (void)dealloc
 
 - (void)invalidate
 {
+  // [TODO: GH#858
+  // ensure observer callbacks do not hold a reference to weak self via pauseCallback
+  for (RCTModuleData *moduleData in _frameUpdateObservers) {
+    id<RCTFrameUpdateObserver> observer = (id<RCTFrameUpdateObserver>)moduleData.instance;
+    [observer setPauseCallback:nil];
+  }
+  [_frameUpdateObservers removeAllObjects];	// just to be explicit
+  // TODO: GH#858]
+
   [_jsDisplayLink invalidate];
 }