Avoid explicit flush when handling persistent sessions

[ahus1]

Description

Looking at JpaUserSessionPersisterProvider, there are several explicit em.flush() statements which hurt performance.

Discussion

No response

Motivation

Since Hibernate 5.2 which was published after the code in question was written, Hibernate changed its behavior to automatically flush before a query, see https://vladmihalcea.com/how-do-jpa-and-hibernate-define-the-auto-flush-mode/

Due to this, it is OK not to flush explictly, and all changes will be in the database before any future delete or select is triggered in the same transaction.

It could be a change in semantics that a conflict of entries becomes visible only once the transaction is committed.

Still, the old code only caught situations where another transaction has already committed that entry, and situations where concurrent transactions occur can still happen, so removing em.flush() shouldn't break anything that wasn't already broken before.

Details

This should not be affected by the orphan removal problem seen in https://hibernate.atlassian.net/browse/HHH-15230