When doing a rolling upgrade, we need to be able to tell all rpc clients
to hold off on sending newer versions of messages until all nodes
understand the new message version. This patch adds the oslo component
of this.
It's quite simple. The rpc proxy just stores the version cap and will
raise an exception if code ever tries to send a message that exceeds
this cap.
Allowing the cap to be configured and generating different types of
messages based on the configured value is the hard part here, but that
is left up to the project using the rpc library.
Implements blueprint rpc-version-control.
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