[core] add support for stale responses

We're now returning stale responses from cache. Those responses will have the `stale` flag set to true. Currently, all requesters in the core code discard stale responses, and cancel the request immediately after they got a non-stale response.

test/storage/http_other_loop.cpp

@@ -16,6 +16,7 @@ TEST_F(Storage, HTTPOtherLoop) {
                [&](const Response &res) {
         fs.cancel(req);
         EXPECT_EQ(Response::Successful, res.status);
+        EXPECT_EQ(false, res.stale);
         EXPECT_EQ("Hello World!", res.data);
         EXPECT_EQ(0, res.expires);
         EXPECT_EQ(0, res.modified);