Update qclient with fixed QSet::sscan, add test

qclient @ 256e5f51

-Subproject commit 40feec72c5885042b65b047407e5fab91bbe9ee3
+Subproject commit 256e5f51720eb543ac9c7f217dd46e2e70e823a0

test/e2e.cc

@@ -759,6 +759,10 @@ TEST_F(Raft_e2e, sscan) {
   ASSERT_EQ(pair.first, "next:c");
   ASSERT_EQ(pair.second, make_vec("a", "b"));
 
+  pair = qset.sscan(pair.first, 2);
+  ASSERT_EQ(pair.first, "next:e");
+  ASSERT_EQ(pair.second, make_vec("c", "d"));
+
   QSet qset2(*tunnel(leaderID), "not-existing");
   pair = qset2.sscan("0", 2);