channeldb/migration: add generic CreateTLB migration

This commit creates a generic migration for creating top-level buckets.

channeldb/migration/create_tlb.go

@@ -0,0 +1,27 @@
+package migration
+
+import (
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/channeldb/kvdb"
+)
+
+// CreateTLB creates a new top-level bucket with the passed bucket identifier.
+func CreateTLB(bucket []byte) func(kvdb.RwTx) error {
+	return func(tx kvdb.RwTx) error {
+		log.Infof("Creating top-level bucket: \"%s\" ...", bucket)
+
+		if tx.ReadBucket(bucket) != nil {
+			return fmt.Errorf("top-level bucket \"%s\" "+
+				"already exists", bucket)
+		}
+
+		_, err := tx.CreateTopLevelBucket(bucket)
+		if err != nil {
+			return err
+		}
+
+		log.Infof("Created top-level bucket: \"%s\"", bucket)
+		return nil
+	}
+}