Bound the WHOIS fetcher's success and error time lists (closes #118)

recordSuccess and recordFailure appended a time on every fetch, and only
GetStats, run when a stats endpoint is requested, dropped the times older
than statsWindow, so a daemon nobody polled kept every one. Each now drops
those times before it appends. The drop takes them off the front of the
list instead of copying the rest: the times are in the order they were
recorded, and it now runs on every fetch.

The fetcher reads the time through a now field so a test can set the
clock. The test records 100,000 successes and failures a second apart
without asking for the stats and finds one hour's worth of each kept.

Model: opus-5-5

internal/routewatch/asnfetcher_internal_test.go

@@ -203,3 +203,35 @@ func TestASNFetcherStopReturnsByItsDeadline(t *testing.T) {
 		t.Fatal("the fetcher goroutine did not return after the lookup ended")
 	}
 }
+
+// TestRecordKeepsOnlyTheStatsWindow records a success and a failure every
+// second, by a clock the test sets, far longer than statsWindow, and never
+// asks for the stats. The fetcher must keep only the times within the last
+// statsWindow.
+func TestRecordKeepsOnlyTheStatsWindow(t *testing.T) {
+	t.Parallel()
+
+	const (
+		records = 100_000
+		step    = time.Second
+		want    = int(statsWindow / step)
+	)
+
+	clock := time.Now()
+
+	f := NewASNFetcher(newMockStore(), logger.New().Logger)
+	f.now = func() time.Time { return clock }
+
+	for range records {
+		clock = clock.Add(step)
+
+		f.recordSuccess()
+		f.recordFailure()
+	}
+
+	if len(f.successTimes) != want || len(f.errorTimes) != want {
+		t.Errorf("%d success and %d error times kept, want %d of each, "+
+			"those of the last %s", len(f.successTimes), len(f.errorTimes),
+			want, statsWindow)
+	}
+}