TomcatEmbeddedServletContainerFactory should not create a StandardManager object unconditionally

[zeodtr]

In TomcatEmbeddedServletContainerFactory.configureSession(), a StandardManager object is created if this.manager is null. But it is not needed if isPersistSession() is false.

This superfluous creation has somewhat bad effect when another manager is needed for the context. For example, when configuring tomcat session clustering, a DeltaManager is needed instead of StandardManager. an application programmer can replace the manager by adding a TomcatContextCustomizer that's called later, but it was not needed on previous versions of Spring Boot (since the manager would be created automatically later) thus breaking compatibility.

On the first commit that's related this issue, it had been properly done - that is, a StandardManager is created only when isPersistSession() is true. But on a commit later, it was changed to current form. I think the first commit's code is better.

[philwebb]

We need to ensure that StandardManager is set so that the disablePersistSession code works. Perhaps we can shift this to a later point.