wtxmgr: separate PutTxLabel into store-independent function

Separate out the logic we use to write labels so that it can be used
in dropwtxmgr to re-add labels if we want to keep them when we drop the
rest of the db.

wtxmgr/tx.go

@@ -984,6 +984,15 @@ func (s *Store) PutTxLabel(ns walletdb.ReadWriteBucket, txid chainhash.Hash,
 		return err
 	}
 
+	return PutTxLabel(labelBucket, txid, label)
+}
+
+// PutTxLabel writes a label for a tx to the bucket provided. Note that it does
+// not perform any validation on the label provided, or check whether there is
+// an existing label for the txid.
+func PutTxLabel(labelBucket walletdb.ReadWriteBucket, txid chainhash.Hash,
+	label string) error {
+
 	// We expect the label length to be limited on creation, so we can
 	// store the label's length as a uint16.
 	labelLen := uint16(len(label))