[ReactNative] Fix RCTJavaScriptContext deallocation

Summary:
The context wasn't being explicitly released before, since it'd be immediately
released. Now that the executors are bridge modules, it was only being deallocated
when the modules were released, what caused the threads to not be released at all.

React/Base/RCTBatchedBridge.m

@@ -347,15 +347,16 @@ RCT_NOT_IMPLEMENTED(-initWithBundleURL:(__unused NSURL *)bundleURL
     RCTLatestExecutor = nil;
   }
 
-  void (^mainThreadInvalidate)(void) = ^{
-    RCTAssertMainThread();
-
   [_mainDisplayLink invalidate];
   _mainDisplayLink = nil;
 
   // Invalidate modules
   dispatch_group_t group = dispatch_group_create();
   for (RCTModuleData *moduleData in _modules) {
+    if (moduleData.instance == _javaScriptExecutor) {
+      continue;
+    }
+
     if ([moduleData.instance respondsToSelector:@selector(invalidate)]) {
       [moduleData dispatchBlock:^{
         [(id<RCTInvalidating>)moduleData.instance invalidate];
@@ -364,36 +365,18 @@ RCT_NOT_IMPLEMENTED(-initWithBundleURL:(__unused NSURL *)bundleURL
     moduleData.queue = nil;
   }
   dispatch_group_notify(group, dispatch_get_main_queue(), ^{
+    [_javaScriptExecutor executeBlockOnJavaScriptQueue:^{
+      [_jsDisplayLink invalidate];
+      _jsDisplayLink = nil;
+
+      [_javaScriptExecutor invalidate];
+      _javaScriptExecutor = nil;
+    }];
+
     _modules = nil;
     _modulesByName = nil;
     _frameUpdateObservers = nil;
   });
-  };
-
-  if (!_javaScriptExecutor) {
-
-    // No JS thread running
-    mainThreadInvalidate();
-    return;
-  }
-
-  [_javaScriptExecutor executeBlockOnJavaScriptQueue:^{
-
-    /**
-     * JS Thread deallocations
-     */
-    [_javaScriptExecutor invalidate];
-    _javaScriptExecutor = nil;
-
-    [_jsDisplayLink invalidate];
-    _jsDisplayLink = nil;
-
-    /**
-     * Main Thread deallocations
-     */
-    dispatch_async(dispatch_get_main_queue(), mainThreadInvalidate);
-
-  }];
 }
 
 #pragma mark - RCTBridge methods

React/Executors/RCTContextExecutor.m

@@ -320,6 +320,7 @@ static NSError *RCTNSErrorFromJSError(JSContextRef context, JSValueRef jsError)
                    onThread:_javaScriptThread
                  withObject:nil
               waitUntilDone:NO];
+  _context = nil;
 }
 
 - (void)dealloc