jsonlog: Ignore log_timezone and use UTC and use the "Z" time zone format

[gsstark]

This is the format the popular json filtering tool, jq, can handle and
it's the format Javascript produces and expects.

[gsstark]

So for my purposes it doesn't matter which of #21 or this one you take. We have log_timezone set to UTC anyways so it will produce the same output.

If you are worried about existing users who have an expectation that log_timezone is respected then take PR 21. It uses numeric offsets (or Z for GMT) so it's still a change for users but at least they get to set log_timezone.

If you are convinced by my argument that JSON is a machine readable format and it's valuable to be able to run jq or other tools on a client site without having to program tools to convert time zones then take this PR.

I really recommend against sticking with the status quo though. Currently there's no log_timezone you can set that will produce timestamps that jq can read since you put a " GMT" at the end of the timestamps. And if you specify another log_timezone then you use a timezone abbreviation that there's no standard unambiguous way to parse at all.

[michaelpq]

I have just pushed this patch as f7960f1. If there are complains in the future, I am also open to a GUC switch which is able to change formats. So let's see later if this introduces problems.