go/zodb: Require DB to be closed

DB is a "handle to database". It has to be closed to release its
resources when no longer needed. We don't have to release anything
currently, but that will soon change in a followup patch.

go/zodb/btree/btree_test.go

@@ -106,6 +106,9 @@ func TestBTree(t *testing.T) {
 		t.Fatal(err)
 	}
 	db := zodb.NewDB(stor)
+	defer func() {
+		err := db.Close(); X(err)
+	}()
 
 	txn, ctx := transaction.New(ctx)
 	defer txn.Abort()

go/zodb/db.go

@@ -65,11 +65,19 @@ type invEntry struct {
 
 
 // NewDB creates new database handle.
+//
+// Created database handle must be closed when no longer needed.
 func NewDB(stor IStorage) *DB {
 	// XXX db options?
 	return &DB{stor: stor}
 }
 
+// Close closes database handle.
+func (db *DB) Close() error {
+	return nil
+}
+
+
 // ConnOptions describes options to DB.Open .
 type ConnOptions struct {
 	At     Tid  // if !0, open Connection bound to `at` view of database; not latest.