webapp: improve pcap download

[yunzheng]

Downloading a PCAP will now limit packets to the flow only.

Before it would just return the LZ4 compressed PCAP file that contains the flow.

[erdnaxe]

Hi! Thank you for this patch.

I definitely agree that it is a cool feature that should be present in Shovel. However, I need more time to think about the proposed implementation:

• LZ4 format seems harder to use for Windows users, so I agree that we should rather serve pcap files.
• I am not a big fan of calling lz4cat then tcpdump in subprocesses.
• I was planning to look into Suricata to produce pcap.gz files directly, things are already moving a bit upstream: https://redmine.openinfosecfoundation.org/issues/8132 (libarchive might come to suricata)
• If we have pcap.gz files, then scapy + gzip modules could be sufficient. This would prevent using subprocess.

In the mean time, I will definitely keep the PR opened to let people cherrypick these commits if they want the feature now.