Keep a trailing AS set out of the origin and the peerings (closes #75)

A path that ends in an AS set of several members, such as
[64496, 64497, [64498, 64499]], now has no origin: its route is stored
with origin 0 (none), and no member of the set is in a peering. A set of
one member counts as that AS. An announcement with an empty path is
still not stored.

ASPath keeps such a set apart from the sequence and gives the origin.
The prefix and AS handlers take the origin from it; the peering handler
uses the sequence only. The stored AS path still lists the set's members
at the end.

The prefix and prefix length pages show origin 0 as no known origin, the
IP lookup asks no WHOIS lookup for it, and the README says what it means.

Model: opus-5-5

README.md

@@ -54,6 +54,11 @@ DEBUG=routewatch ./routewatch
 - `GET /api/v1/as/{asn}` - Get prefixes announced by an AS
 - `GET /api/v1/prefix/{prefix}` - Get routes for a specific prefix
 
+An origin AS of 0 (`origin_asn` in a route, `asn` in an IP lookup, the key `0`
+in a prefix's `origins`) means the route has no known origin: its AS path ends
+in an AS set of several members, any of which may have originated it. The pages
+show such a route as having no known origin.
+
 ## Code Structure
 
 ```