watchtower: add filter function to ListTowers

And then only load active towers on client start up.

watchtower/wtclient/client.go

@@ -261,7 +261,10 @@ func getTowerAndSessionCandidates(db DB, keyRing ECDHKeyRing,
 	opts ...wtdb.ClientSessionListOption) (
 	map[wtdb.SessionID]*ClientSession, error) {
 
-	towers, err := db.ListTowers()
+	// Fetch all active towers from the DB.
+	towers, err := db.ListTowers(func(tower *wtdb.Tower) bool {
+		return tower.Status == wtdb.TowerStatusActive
+	})
 	if err != nil {
 		return nil, err
 	}

watchtower/wtclient/interface.go

@@ -41,8 +41,9 @@ type DB interface {
 	LoadTowerByID(wtdb.TowerID) (*wtdb.Tower, error)
 
 	// ListTowers retrieves the list of towers available within the
-	// database.
-	ListTowers() ([]*wtdb.Tower, error)
+	// database. The filter function may be set in order to filter out the
+	// towers to be returned.
+	ListTowers(filter wtdb.TowerFilterFn) ([]*wtdb.Tower, error)
 
 	// NextSessionKeyIndex reserves a new session key derivation index for a
 	// particular tower id and blob type. The index is reserved for that

watchtower/wtclient/manager.go

@@ -455,7 +455,7 @@ func (m *Manager) Stats() ClientStats {
 func (m *Manager) RegisteredTowers(opts ...wtdb.ClientSessionListOption) (
 	map[blob.Type][]*RegisteredTower, error) {
 
-	towers, err := m.cfg.DB.ListTowers()
+	towers, err := m.cfg.DB.ListTowers(nil)
 	if err != nil {
 		return nil, err
 	}