Added pcap microseconds field to packet and added logic to use UDP packet length

- Added pcap timestamp microseconds field "tmicros" to complement the
current timestamp in seconds field "ts"
- Added logic to read the packet length field in UDP packets and use it
as a primary reference to extract payload. Before this patch, the code
assumed that the payload is the remainder of the packet after the
headers. However that is not the case when pcap packets are padded.