Add autorelease pool for each run loop for JS Thread (facebook#27395)

Summary:
Fixes facebook#27327 , we need to create autorelease pool for each run loop in secondary thread, otherwise application may have memory issues. More details can refer to [CreatingThreads](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Multithreading/CreatingThreads/CreatingThreads.html)
![image](https://user-images.githubusercontent.com/5061845/70033738-05fa2980-15eb-11ea-9adb-f01bee937766.png)

## Changelog

[iOS] [Fixed] - Add autorelease pool for each run loop for JS Thread
Pull Request resolved: facebook#27395

Test Plan: Example can be found in facebook#27327. No memory spikes any more.

Reviewed By: PeteTheHeat

Differential Revision: D19132504

Pulled By: fkgozali

fbshipit-source-id: d1747f27d36e9a7934966b34aa46d344e06193b3

Author: zhongwuzw

React/CxxBridge/RCTMessageThread.mm

@@ -36,7 +36,12 @@
 
 // This is analogous to dispatch_async
 void RCTMessageThread::runAsync(std::function<void()> func) {
-  CFRunLoopPerformBlock(m_cfRunLoop, kCFRunLoopCommonModes, ^{ func(); });
+  CFRunLoopPerformBlock(m_cfRunLoop, kCFRunLoopCommonModes, ^{
+    // Create an autorelease pool each run loop to prevent memory footprint from growing too large, which can lead to performance problems.
+    @autoreleasepool {
+      func();
+    }
+  });
   CFRunLoopWakeUp(m_cfRunLoop);
 }