watchtower: use a stable blob identifier

In this commit, we add an Identifier method to the blob.Type struct
which returns a unique identifier for a given blob type. This identifier
is then used for initialising the disk overflow queue of the given
client.

watchtower/wtclient/client.go

@@ -350,10 +350,12 @@ func New(config *Config) (*TowerClient, error) {
 		cfg.WriteTimeout = DefaultWriteTimeout
 	}
 
-	prefix := "(legacy)"
-	if cfg.Policy.IsAnchorChannel() {
-		prefix = "(anchor)"
+	identifier, err := cfg.Policy.BlobType.Identifier()
+	if err != nil {
+		return nil, err
 	}
+	prefix := fmt.Sprintf("(%s)", identifier)
+
 	plog := build.NewPrefixLog(prefix, log)
 
 	// Load the sweep pkscripts that have been generated for all previously
@@ -363,10 +365,7 @@ func New(config *Config) (*TowerClient, error) {
 		return nil, err
 	}
 
-	var (
-		policy  = cfg.Policy.BlobType.String()
-		queueDB = cfg.DB.GetDBQueue([]byte(policy))
-	)
+	queueDB := cfg.DB.GetDBQueue([]byte(identifier))
 	queue, err := NewDiskOverflowQueue[*wtdb.BackupID](
 		queueDB, cfg.MaxTasksInMemQueue, plog,
 	)