Fixed packet timestamp code by using 'raw' instead of 'show' value

sniff_time() expects a float in sniff_timestamp, but sniff_timestamp was using the 'show' value from the tshark XML output, which contains an interpreted datetime string. Fixed this by getting the value for sniff_timestamp from the 'raw' field, which contains the timestamp as a float in string format. This is converted to float in-place in sniff_time()