[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

@@ -347,53 +347,36 @@ RCT_NOT_IMPLEMENTED(-initWithBundleURL:(__unused NSURL *)bundleURL
     RCTLatestExecutor = nil;
   }
 
-  void (^mainThreadInvalidate)(void) = ^{
-    RCTAssertMainThread();
+  [_mainDisplayLink invalidate];
+  _mainDisplayLink = nil;
 
-    [_mainDisplayLink invalidate];
-    _mainDisplayLink = nil;
-
-    // Invalidate modules
-    dispatch_group_t group = dispatch_group_create();
-    for (RCTModuleData *moduleData in _modules) {
-      if ([moduleData.instance respondsToSelector:@selector(invalidate)]) {
-        [moduleData dispatchBlock:^{
-          [(id<RCTInvalidating>)moduleData.instance invalidate];
-        } dispatchGroup:group];
-      }
-      moduleData.queue = nil;
+  // Invalidate modules
+  dispatch_group_t group = dispatch_group_create();
+  for (RCTModuleData *moduleData in _modules) {
+    if (moduleData.instance == _javaScriptExecutor) {
+      continue;
     }
-    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
-      _modules = nil;
-      _modulesByName = nil;
-      _frameUpdateObservers = nil;
-    });
-  };
 
-  if (!_javaScriptExecutor) {
-
-    // No JS thread running
-    mainThreadInvalidate();
-    return;
+    if ([moduleData.instance respondsToSelector:@selector(invalidate)]) {
+      [moduleData dispatchBlock:^{
+        [(id<RCTInvalidating>)moduleData.instance invalidate];
+      } dispatchGroup:group];
+    }
+    moduleData.queue = nil;
   }
+  dispatch_group_notify(group, dispatch_get_main_queue(), ^{
+    [_javaScriptExecutor executeBlockOnJavaScriptQueue:^{
+      [_jsDisplayLink invalidate];
+      _jsDisplayLink = nil;
 
-  [_javaScriptExecutor executeBlockOnJavaScriptQueue:^{
+      [_javaScriptExecutor invalidate];
+      _javaScriptExecutor = nil;
+    }];
 
-    /**
-     * JS Thread deallocations
-     */
-    [_javaScriptExecutor invalidate];
-    _javaScriptExecutor = nil;
-
-    [_jsDisplayLink invalidate];
-    _jsDisplayLink = nil;
-
-    /**
-     * Main Thread deallocations
-     */
-    dispatch_async(dispatch_get_main_queue(), mainThreadInvalidate);
-
-  }];
+    _modules = nil;
+    _modulesByName = nil;
+    _frameUpdateObservers = nil;
+  });
 }
 
 #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