DATAJPA-1574 - Add support for saveAllAndFlush.

[skrabbenborg]

Closes #1883

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[skrabbenborg]

Would it be advisable to add a statement in the javadoc that saveAll() and flush() are used, thus overriding those methods will also change the functionality of this method?

[mp911de]

We typically don't document these things. It gives us a certain freedom to change the actual implementation. You could add @see #saveall and @see #flush references.

[schauder]

Thanks for your work. That's polished and merged.