Enable packet capture for debugging

Traffic on DPDK ports can be captured and displayed for debugging purposes:

https://doc.dpdk.org/guides/howto/packet_capture_framework.html

We should investigate whether Gatekeeper can support this feature, either by default or in a debugging mode.

Some relevant notes from the page above:

> The dpdk-pdump tool can only be used in conjunction with a primary application which has the packet capture framework initialized already.
>
> The dpdk-pdump tool depends on the libpcap based PMD.

This could be useful for issues like #451.