Re: Add new protocol message to change GUCs for usage with future protocol-only GUCs

New patchset attached, where I split up the patches in smaller logical units.
Note that the first 4 patches in this series are not making any
protocol changes. All they do is set up infrastructure in the code
that allows us to make protocol changes in the future.

I hope that those 4 should all be fairly uncontroversial, especially
patch 1 seems a no-brainer to me. Note that this infrastructure would
be needed for any patchset that introduces protocol changes.

The 5th only bumps the version

The 6th introduces the _pq_.protocol_managed_parms protocol parameter

The 7th adds the new ParameterSet protocol message