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/wtclient/client.go

@@ -206,7 +206,7 @@ func New(config *Config) (*TowerClient, error) {
 	// use any of these session if their policies match the current policy
 	// of the client, otherwise they will be ignored and new sessions will
 	// be requested.
-	sessions, err := cfg.DB.ListClientSessions()
+	sessions, err := cfg.DB.ListClientSessions(nil)
 	if err != nil {
 		return nil, err
 	}