Switch to non-blocking thread after WebSession creation

See gh-26958

spring-web/src/main/java/org/springframework/web/server/session/InMemoryWebSessionStore.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -118,7 +118,8 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 		this.expiredSessionChecker.checkIfNecessary(now);
 
 		return Mono.<WebSession>fromSupplier(() -> new InMemoryWebSession(now))
-				.subscribeOn(Schedulers.boundedElastic());
+				.subscribeOn(Schedulers.boundedElastic())
+				.publishOn(Schedulers.parallel());
 	}
 
 	@Override