Re: Emit a warning if the extension's GUC is set incorrectly

I wrote:
> As a stopgap to turn the farm green again, I am going to revert
> 75d22069e as well as my followup patches.  If we don't want to
> give up on that idea altogether, we have to find some way to
> suppress the chatter from parallel workers.  I wonder whether
> it would be appropriate to go further than we have, and actively
> delete placeholders that turn out to be within an extension's
> reserved namespace.  The core issue here is that workers don't
> necessarily set GUCs and load extensions in the same order that
> their parent did, so if we leave any invalid placeholders behind
> after reserving an extension's prefix, we're risking issues
> during worker start.

Here's a delta patch (meant to be applied after reverting cab5b9ab2)
that does things like that.  It fixes the parallel-mode problem ...
so do we want to tighten things up this much?

                        regards, tom lan