StatelessSession, collections, and second-level cache

[gavinking]

In Hibernate 7, StatelessSession gained quite a lot of new functionality, in particular:

1. the ability to fully handle collections, and
2. the ability to interact with the second-level cache.
3. Also, the default JDBC batch size changed.

These changes need to be replicated in ReactiveStatelessSessionImpl.

This really needs to happen before Hibernate Reactive 3.0 goes final, because these are major changes to the behavior of the StatelessSession that can in principle break existing programs.

Also, it's probably time to just go through both ReactiveStatelessSessionImpl and plain ReactiveSessionImpl and audit them for divergences from Hibernate ORM 7.