Improve WebSocketServer Error Handling (#6230)

* Improve WebSocketServer Error Handling

Closes: https://github.com/parse-community/parse-server/issues/6173

Prevents an unhandled server rejection.

Includes an example for LiveQuery test and closing the proper connections.

Improve live query monitoring

* fix tests

spec/ParseLiveQuery.spec.js

@@ -0,0 +1,35 @@
+'use strict';
+
+describe('ParseLiveQuery', function() {
+  it('can subscribe to query', async done => {
+    await reconfigureServer({
+      liveQuery: {
+        classNames: ['TestObject'],
+      },
+      startLiveQueryServer: true,
+      verbose: false,
+      silent: true,
+    });
+    const object = new TestObject();
+    await object.save();
+
+    const query = new Parse.Query(TestObject);
+    query.equalTo('objectId', object.id);
+    const subscription = await query.subscribe();
+    subscription.on('update', async object => {
+      expect(object.get('foo')).toBe('bar');
+      done();
+    });
+    object.set({ foo: 'bar' });
+    await object.save();
+  });
+
+  afterEach(async function(done) {
+    const client = await Parse.CoreManager.getLiveQueryController().getDefaultLiveQueryClient();
+    client.close();
+    // Wait for live query client to disconnect
+    setTimeout(() => {
+      done();
+    }, 1000);
+  });
+});