lib/connections, lib/model: Track last connection duration (ref #7223) (#7242)

This adds a statistic to track the last connection duration per device.
It isn't used for much in this PR, but it's available for #7223 to use
in deciding how to order device connection attempts (deprioritizing
devices that just dropped our connection the last time).

lib/stats/stats_test.go

@@ -0,0 +1,43 @@
+// Copyright (C) 2020 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// The existence of this file means we get 0% test coverage rather than no
+// test coverage at all. Remove when implementing an actual test.
+
+package stats
+
+import (
+	"testing"
+	"time"
+
+	"github.com/syncthing/syncthing/lib/db/backend"
+	"github.com/syncthing/syncthing/lib/protocol"
+)
+
+func TestDeviceStat(t *testing.T) {
+	db := backend.OpenLevelDBMemory()
+	defer db.Close()
+
+	sr := NewDeviceStatisticsReference(db, protocol.LocalDeviceID)
+	if err := sr.WasSeen(); err != nil {
+		t.Fatal(err)
+	}
+	if err := sr.LastConnectionDuration(42 * time.Second); err != nil {
+		t.Fatal(err)
+	}
+
+	stat, err := sr.GetStatistics()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if d := time.Since(stat.LastSeen); d > 5*time.Second {
+		t.Error("Last seen far in the past:", d)
+	}
+	if d := stat.LastConnectionDurationS; d != 42 {
+		t.Error("Bad last duration:", d)
+	}
+}