BGP bestpath algorithm should more closely follow RFC 4721

[taspelund]

The current BGP bestpath algorithm does not implement the full Phase 2 tie breaking rules from RFC 4721.
When the BESTPATH_FANOUT is set to 1 (which is currently a const, but won't always be), we don't select the singular best path via the full tie-breaking process outlined in the spec.

Some quick (but not exhaustive) examples are:

• we don't look at the origin attribute
• we don't look at the peer's Router-ID
• we don't look at the IP address we use to connect to the peer
• we don't pick the oldest path if all else is the same

This is something we should do to bring us into better alignment with the standards, and so that there's a little more determinism in which path gets picked in non multipath scenarios

[taspelund]

I have a branch with these changes here that depend on #526

Once we address the issues causing CI to fail we should be able to get #526 merged and this can be evaluated.