Guard transport metadata against raw byte payloads

[keilogic]

Summary

• leave raw byte payloads unclassified in TransportMeta instead of treating them as transport objects
• read parsed transport payloads from the dpkt object's data attribute
• add regression coverage for raw byte payloads and normal TCP metadata

Fixes #10.

Verification

• py -m py_compile chains\links\transport_meta.py chains\links\transport_meta_test.py
• py -m pytest chains\links\transport_meta_test.py -q
• py -m flake8 chains\links\transport_meta.py chains\links\transport_meta_test.py
• git diff --check