feat: Asynchronous initialization of Parse Server (#8232)

BREAKING CHANGE: This release introduces the asynchronous initialization of Parse Server to prevent mounting Parse Server before being ready to receive request; it changes how Parse Server is imported, initialized and started; it also removes the callback `serverStartComplete`; see the [Parse Server 6 migration guide](https://github.com/parse-community/parse-server/blob/alpha/6.0.0.md) for more details (#8232)

spec/support/FailingServer.js

@@ -6,14 +6,20 @@ const ParseServer = require('../../lib/index').ParseServer;
 
 const databaseURI = 'mongodb://doesnotexist:27017/parseServerMongoAdapterTestDatabase';
 
-ParseServer.start({
-  appId: 'test',
-  masterKey: 'test',
-  databaseAdapter: new MongoStorageAdapter({
-    uri: databaseURI,
-    mongoOptions: {
-      serverSelectionTimeoutMS: 2000,
-    },
-  }),
-  filesAdapter: new GridFSBucketAdapter(databaseURI),
-});
+(async () => {
+  try {
+    await ParseServer.startApp({
+      appId: 'test',
+      masterKey: 'test',
+      databaseAdapter: new MongoStorageAdapter({
+        uri: databaseURI,
+        mongoOptions: {
+          serverSelectionTimeoutMS: 2000,
+        },
+      }),
+      filesAdapter: new GridFSBucketAdapter(databaseURI),
+    });
+  } catch (e) {
+    process.exit(1);
+  }
+})();