Return saved entity reference instead of original reference.

[petitcl]

Make SimpleReactiveMongoRepository#saveAll(Publisher<S>) return the saved entity references instead of the original references.

Closes #3609

This is my first pull request, so I am sorry if am missing anything. Please tell me if something is missing and I will add it gladly.

I added a ReactiveImmutablePersonRepository and ImmutablePerson classes in SimpleReactiveMongoRepositoryTests but it would probably be better to create a dedicated test class that validates that immutable uses cases (including this one) are working.

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[mp911de]

Thanks a lot. We'll take your pull request from here.

[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.