Switching to template(type="list") declarations for the message format
templates makes them significantly easier to read and maintain, but
the did become *much* more verbose.  Still a win.

At the same time, we remove one unused template (LogFmtIsoTime) and
rename the two remaining (LogFmtSpaceTime to NSCLogFormat, and
LogFmtJoinedLog to NSCLogFormatSourcePri).