Re: Support for N synchronous standby servers - take 2

OK, so this is leading us to the following points:
- Use a JSON object to define the quorum/priority groups for the sync state.
- Store it as a GUC, and use the check hook to validate its format,
which is what we have now with s_s_names
- Rely on SIGHUP to maintain an in-memory image of the quorum/priority
sync state
- Have the possibility to define group labels in this JSON blob, and
be able to use those labels in a quorum or priority sync definition.

- For backward-compatibility, use for example s_s_names = 'json' to
switch to the new system.

Also, as a first step of the implementation, do we actually need a set
of functions to manipulate the JSON blob. I mean, we could perhaps
have them in contrib/ but they do not seem mandatory as long as we
document correctly how to document a label group and define a quorum
or priority group, no?