Since nginx only uses 1 second precision for the error logs and syslog
timestamps, we can use receive time on the syslog-ng side. We can switch
to source time once nginx adds RFC 5424 support which is currently in an
open pull request but will likely require changes to add a configuration
option for it. Our approach to working around this within nginx doesn't
work perfectly since $msec generates the time on-demand separately from
the timestamp used by $time_iso8601.
