database: Fix to a locking issue with PostgreSQL

It appears that preparing an INSERT statement on PostgreSQL actually makes it expecting to receive INSERTs and thus, it create some kind of locks for it. If instead, you only send him DELETE statements, it will indefinitely wait for an INSERT and hung.

Godeps/Godeps.json

@@ -42,7 +42,7 @@
 		{
 			"ImportPath": "github.com/google/cayley",
 			"Comment": "v0.4.1-160-gcdf0154",
-			"Rev": "cfbc0b364910ab69bbced5d2f7913ee7e80a0c80"
+			"Rev": "2f5ce909a242795d82529658ceb586863f5dd559"
 		},
 		{
 			"ImportPath": "github.com/julienschmidt/httprouter",

vendor/github.com/google/cayley/graph/sql/quadstore.go

@@ -192,16 +192,10 @@ func (qs *QuadStore) runTxPostgres(tx *sql.Tx, in []graph.Delta, opts graph.Igno
 		return qs.copyFrom(tx, in)
 	}
 
-	insert, err := tx.Prepare(`INSERT INTO quads(subject, predicate, object, label, id, ts, subject_hash, predicate_hash, object_hash, label_hash) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10)`)
-	defer insert.Close()
-	if err != nil {
-		glog.Errorf("Cannot prepare insert statement: %v", err)
-		return err
-	}
 	for _, d := range in {
 		switch d.Action {
 		case graph.Add:
-			_, err := insert.Exec(
+			_, err := tx.Exec(`INSERT INTO quads(subject, predicate, object, label, id, ts, subject_hash, predicate_hash, object_hash, label_hash) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10);`,
 				d.Quad.Subject,
 				d.Quad.Predicate,
 				d.Quad.Object,
@@ -214,7 +208,7 @@ func (qs *QuadStore) runTxPostgres(tx *sql.Tx, in []graph.Delta, opts graph.Igno
 				hashOf(d.Quad.Label),
 			)
 			if err != nil {
-				glog.Errorf("couldn't prepare INSERT statement: %v", err)
+				glog.Errorf("couldn't exec INSERT statement: %v", err)
 				return err
 			}
 		case graph.Delete: