Restore segments to path graph & bug fixes

[mwfarb]

The major work of this PR is to restore segments to the path graph:

• copy in pace and read path database for segment data
• render segment data in UI list of interfaces and paths graph
• filter "dirty" list of segments from paths db against known paths
• use filtered segments to mark proper IAs core/not core

Additional minor fixes and improvements included:

• sort "enp" interfaces first, then "lo", then alphabetically (prevents docker0 loading as default when testing locally)
• reduce logging debug output for webapp.log
• fixed several small runtime errors
• prevented accidental db downgrade
• updated AS Topology graph to consider each unique ip/port as a single border router, which may have multiple interfaces
• use a testing IA when testing on local topology and $SC is undefined

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This change is 

[juagargi]

Not from this PR, but I've noticed we have entries such as `ffaa:1:f` and other `ffaa:1:XXXX` in the `webapp/config/servers_default.json` file. Maybe we can remove those user ASes from the list? Or what was the reason to have them there?

Reviewed 8 of 10 files at r1, 1 of 2 files at r2, 3 of 3 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved

[mwfarb]

The default server addresses come from the addresses we encourage people to test scionlab apps with in our tutorials:
https://netsec-ethz.github.io/scion-tutorials/sample_projects/bwtester/
https://netsec-ethz.github.io/scion-tutorials/sample_projects/access_camera/
https://netsec-ethz.github.io/scion-tutorials/sample_projects/fetch_sensor_readings/

We use servers_default.json to populate the server dropdown list on the webapp Apps tab to make it easier to use out of the box..

Reviewable status: complete! all files reviewed, all discussions resolved

[mwfarb]

As a suggestion to keep them out of the code base, I can add a future PR to migrate these locations to our Visualization GAE tool storing API keys. I probably need to move it anyway so that it moves from scion-viz to scion-apps.

Reviewable status: complete! all files reviewed, all discussions resolved