watchtower: add ClientSessionFilterFn to session requests

In this commit, a new ClientSessionFilterFn parameter is added to the
DB's ListClientSession method which can be used to allow the caller to
specify a filter function for filtering sessions read from the DB.
Currently all filtering of sessions are done after the sessions have
been read from the DB, so adding this option should provide some
efficiency.

watchtower/wtmock/client_db.go

@@ -83,7 +83,7 @@ func (m *ClientDB) CreateTower(lnAddr *lnwire.NetAddress) (*wtdb.Tower, error) {
 		tower = m.towers[towerID]
 		tower.AddAddress(lnAddr.Address)
 
-		towerSessions, err := m.listClientSessions(&towerID)
+		towerSessions, err := m.listClientSessions(&towerID, nil)
 		if err != nil {
 			return nil, err
 		}
@@ -135,7 +135,7 @@ func (m *ClientDB) RemoveTower(pubKey *btcec.PublicKey, addr net.Addr) error {
 		return nil
 	}
 
-	towerSessions, err := m.listClientSessions(&tower.ID)
+	towerSessions, err := m.listClientSessions(&tower.ID, nil)
 	if err != nil {
 		return err
 	}
@@ -220,18 +220,20 @@ func (m *ClientDB) MarkBackupIneligible(_ lnwire.ChannelID, _ uint64) error {
 // optional tower ID can be used to filter out any client sessions in the
 // response that do not correspond to this tower.
 func (m *ClientDB) ListClientSessions(tower *wtdb.TowerID,
+	filterFn wtdb.ClientSessionFilterFn,
 	opts ...wtdb.ClientSessionListOption) (
 	map[wtdb.SessionID]*wtdb.ClientSession, error) {
 
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	return m.listClientSessions(tower, opts...)
+	return m.listClientSessions(tower, filterFn, opts...)
 }
 
 // listClientSessions returns the set of all client sessions known to the db. An
 // optional tower ID can be used to filter out any client sessions in the
 // response that do not correspond to this tower.
 func (m *ClientDB) listClientSessions(tower *wtdb.TowerID,
+	filterFn wtdb.ClientSessionFilterFn,
 	opts ...wtdb.ClientSessionListOption) (
 	map[wtdb.SessionID]*wtdb.ClientSession, error) {
 
@@ -246,6 +248,11 @@ func (m *ClientDB) listClientSessions(tower *wtdb.TowerID,
 		if tower != nil && *tower != session.TowerID {
 			continue
 		}
+
+		if filterFn != nil && !filterFn(&session) {
+			continue
+		}
+
 		sessions[session.ID] = &session
 
 		if cfg.PerMaxHeight != nil {