multi: add reset closure to kvdb.Update

Similarly as with kvdb.View this commits adds a reset closure to the
kvdb.Update call in order to be able to reset external state if the
underlying db backend needs to retry the transaction.

channeldb/migtest/migtest.go

@@ -47,7 +47,7 @@ func ApplyMigration(t *testing.T,
 
 	// beforeMigration usually used for populating the database
 	// with test data.
-	err = kvdb.Update(cdb, beforeMigration)
+	err = kvdb.Update(cdb, beforeMigration, func() {})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -65,14 +65,14 @@ func ApplyMigration(t *testing.T,
 
 		// afterMigration usually used for checking the database state and
 		// throwing the error if something went wrong.
-		err = kvdb.Update(cdb, afterMigration)
+		err = kvdb.Update(cdb, afterMigration, func() {})
 		if err != nil {
 			t.Fatal(err)
 		}
 	}()
 
 	// Apply migration.
-	err = kvdb.Update(cdb, migrationFunc)
+	err = kvdb.Update(cdb, migrationFunc, func() {})
 	if err != nil {
 		t.Logf("migration error: %v", err)
 	}