Support pcap with nano-second resolution

[pstavirs]

Currently Ostinato only supports millisecond resolution pcap files.

Pcap file format reference

[pstavirs]

Fix for #83 will resolve this as well - albeit with the caveat that the nano-second resolution is lost

[pstavirs]

Reopening to add native support for nanosecond pcap for the following cases -

• Native support for reading nanosecpcap directly (non via-pdml)
• Native support for reading nanosecpcap via-pdml (already supported: PDML only supports nanosec)
• If via-pdml is set, convert non-pcap (e.g. pcapng) directly to PDML - this is happening today unintentionally - make it intentional (PDML needs PCAP file to cross check packet bytes, so direct conversion not possible)
• When converting non-pcap to pcap, use nsecpcap format
• Native support for saving to nanosecpcap
• Capture support in nanosec precision

In all above cases, ensure that timestamps are read/written correctly for accurate replay