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)
* Changing __indexBuildCompletionCallbackForTests callback to serverStartComplete
* Improving serverStartComplete callback to avoid production unhandled promise rejection
* Add test to check inexistence of unhandled promise rejection on server fail
* Removing some hooks delays
* Removing delay after reconfigureServer
* Improving code style
