Store the routing table in the database (part of #122)

A new routes table holds one row per prefix and origin AS: the peer set
as the bitset's bytes, and the two kept routes, each with its peer's
number, AS path and time in seconds. A new route_peers table holds each
peer's number and address.

WriteRouteChanges writes one TakeChanges result in a transaction.
ReadRouteTable reads both tables into a new routing table, through the
new Table.Load, which records no change. It fails on a row that names a
peer number missing from route_peers. Nothing uses them yet.

Model: opus-5-5

internal/database/interface.go

@@ -1,5 +1,7 @@
 // Package database provides SQLite storage for BGP routing data including ASNs,
-// prefixes, announcements, peerings, and live route tables.
+// prefixes, announcements, peerings, and live route tables. Its routes table
+// holds a copy of the whole routing table that internal/routetable keeps in
+// memory.
 package database
 
 import (