Functions with Flux and Message (as well as POJOs and Flux of POJO
which were already supported) should now work if they are created in
an isolated class loader. Preconditions:
* The class loaders must have the reactor-core (and reactive-streams)
shared between the app and the function. Practically speaking this means
there has to be a parent class loader with just reactive types, and
sibling children for the app and the function. This is not a new
requirement (it was needed for Flux of POJO anyway).
* Message types are handled reflectively, so they don't have to be in a
shared class loader. But they do have to be on the class path on
both sides (obviously).
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