RIB stats & AS Set flatten scripts

[SichangHe]

• Crate rib_stats to generate AS + AS pair + route stats for All downloaded RIBs from download_ribs.py.
  • File formatting: <collector name>--<stats name>.csv where <stats name> is route_stats, as_stats, or as_pair_stats.
  • Write route stats CSV files as the report generation happens so we have enough RAM.
• Make bgp.stats functions take borrowed Report for reuse across different stats generation.
• Evcxr script to flatten each AS Set to its known members.
  • Ignore unrecorded AS Set members.
  • Stupid single-pass recursive depth-first search (fast enough).
  • Output
    • as_sets.txt: each line <AS Set name>:<member ASN-0>,<member ASN-1>…
    • as_set_sizes.csv, columns: as_set, size.

Next step: Python script to make use of the generated RIB stats.