Syslogwriter doesn't include fractional seconds

Including fractional seconds is important for getting the correct ordering of logging events insyslog collectors.

RFC 5424 allows for microsecond precision however, syslogwriter is using RFC 3339 formatted time which does not include fractional seconds. Go provides the time format RCF339Nano but this is too precise for RFC 5424 formatted syslog messages.

syslogwriter should use a custom time format that uses microsecond precision.