Defer background pre-init till the environment is prepared

Previously, background pre-init was started in response to the
application starting event. This meant that it would be running while
the logging turbo filter was in place and was rejecting all logging.
As a result, any logging performed during pre-init would be lost.

This commit changes background pre-init so that it is now started
in response to the application environment prepared event. By this
point, the logging turbo filter has been removed as the logging levels
have been configured from the environment.

Closes gh-21897

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/BackgroundPreinitializer.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,9 +23,9 @@ import java.util.concurrent.atomic.AtomicBoolean;
 import javax.validation.Configuration;
 import javax.validation.Validation;
 
+import org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent;
 import org.springframework.boot.context.event.ApplicationFailedEvent;
 import org.springframework.boot.context.event.ApplicationReadyEvent;
-import org.springframework.boot.context.event.ApplicationStartingEvent;
 import org.springframework.boot.context.event.SpringApplicationEvent;
 import org.springframework.boot.context.logging.LoggingApplicationListener;
 import org.springframework.context.ApplicationListener;
@@ -66,7 +66,7 @@ public class BackgroundPreinitializer implements ApplicationListener<SpringAppli
 	@Override
 	public void onApplicationEvent(SpringApplicationEvent event) {
 		if (!Boolean.getBoolean(IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME)
-				&& event instanceof ApplicationStartingEvent && multipleProcessors()
+				&& event instanceof ApplicationEnvironmentPreparedEvent && multipleProcessors()
 				&& preinitializationStarted.compareAndSet(false, true)) {
 			performPreinitialization();
 		}