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

Fixed some conflicts again.

To summarize the current state of this patchset, it consists of 5
different parts that could be merged separately:
1. 0001-0005 are needed for any protocol change, and hopefully
shouldn't require much discussion
2. 0006-0009 introduce a new protocol message that can be used to
update GUCs. I think we might want some discussion on the design of
the protocol message, but I think the behaviour has all feedback
incorporated.
3. 0010 Adds GUC contexts for protocol extensions, in the way that Tom
Lane suggested.
4. 0011 Adds a way to mark some GUCs as only changeable using protocol
messages, there's still some discussion needed on this (I can remove
this from the patchset if that makes it easier).
5. 0012 Adds some additional tests for some of the previous features
6. 0013 Add a protocol parameter to control which GUCs have
GUC_REPORT. Quite straightforward imho.