watchtower: extend client db to filter sessions for a specific tower

This currently takes O(N) time as there does not exist an index of
active client sessions for each watchtower within the client's database.
This index is likely to be added in the future.

watchtower/wtmock/client_db.go

@@ -86,13 +86,20 @@ func (m *ClientDB) MarkBackupIneligible(chanID lnwire.ChannelID, commitHeight ui
 	return nil
 }
 
-// ListClientSessions returns the set of all client sessions known to the db.
-func (m *ClientDB) ListClientSessions() (map[wtdb.SessionID]*wtdb.ClientSession, error) {
+// 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) (map[wtdb.SessionID]*wtdb.ClientSession, error) {
+
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
 	sessions := make(map[wtdb.SessionID]*wtdb.ClientSession)
 	for _, session := range m.activeSessions {
+		if tower != nil && *tower != session.TowerID {
+			continue
+		}
 		sessions[session.ID] = session
 	}