Implement listing available prefixes

[9er]

Right now, the tool only returns one prefix (the longest prefix match for a given address, or an exact prefix match for a given prefix with prefix length).

Another neat feature would be returning lists of prefixes. For most use cases this only makes sense if the returned prefixes match certain criteria, like

• return all prefixes with a given origin ASN
• return all prefixes with a given ASN in the as-path
• return all more-specifics for a given prefix and prefix length

Work needed:

• maybe rename the lookup methods in routeinfo/routinfo.go to something like lookupPrefix
• implement some functions like GetMoreSpecificPrefixes or ListPrefixesWithOriginASN
• implement according HTTP API methods in cmd/routeinfo_server
• document HTTP API changes (if Document HTTP API for routeinfo_server #11 is finished)
• implement client-side functionality in lookingglass.js