make has faster

We don't actually need to *load* the value to see if we have it.

datastore.go

@@ -149,6 +149,13 @@ func (d *Datastore) Has(key ds.Key) (bool, error) {
 	return txn.Has(key)
 }
 
+func (d *Datastore) GetSize(key ds.Key) (int, error) {
+	txn := d.newImplicitTransaction(true)
+	defer txn.Discard()
+
+	return txn.Has(key)
+}
+
 func (d *Datastore) Delete(key ds.Key) error {
 	txn := d.newImplicitTransaction(false)
 	defer txn.Discard()
@@ -242,15 +249,15 @@ func (t *txn) Get(key ds.Key) ([]byte, error) {
 }
 
 func (t *txn) Has(key ds.Key) (bool, error) {
-	_, err := t.Get(key)
-
-	if err == nil {
-		return true, nil
-	} else if err == ds.ErrNotFound {
+	_, err := t.txn.Get(key.Bytes())
+	switch err {
+	case badger.ErrKeyNotFound:
 		return false, nil
+	case nil:
+		return true, nil
+	default:
+		return false, err
 	}
-
-	return false, err
 }
 
 func (t *txn) Delete(key ds.Key) error {