Resolve PTR names over DNS-over-HTTPS and cache them for the record TTL (closes #238)

`internal/ptr` asks Google, Cloudflare, AdGuard and Quad9 in turn, 2 seconds in all per address. It keeps answers in memory and writes them through to `cache.db`, opened by the new `internal/cachedb`. A reserved address is not sent to the resolvers.

Judgement calls:
- Each resolver gets an equal share of the time left, so a hung resolver does not use up the 2 seconds.
- The negative TTL is the smaller of the SOA minimum and the SOA's own TTL (RFC 2308).
- Names that are not host names are dropped, as `net.Resolver.LookupAddr` does.

Model: opus-5-5

internal/server/handlers_table_internal_test.go

@@ -85,7 +85,7 @@ func TestIPLookupGivesTheOriginMostPeersCarry(t *testing.T) {
 	t.Parallel()
 
 	s := newServerWithTable(t, newTwoOriginTable())
-	s.resolver = &fakeResolver{}
+	useFakeResolver(s, &fakeResolver{})
 
 	firstSeen := testRouteTime(-1)