If the incoming message has a stream_routekey header, we use that
to route the message to a named function. Also adding the header
to messages coming out of suppliers.
The biggest change here is sort of orthogonal: it fixes a bug where
Spring Integration would subscribe twice to the same input channel
if the FunctionCatalog contains both functions and consumers. Then
when a message comes in it is dispatched to one or the other, but not
both. So the routing key couldn't work without fixing that
problem.
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