[core] set a timer and auto-refresh requests when they expire

When we get a request with an explicit expiration time, we're now starting a timer, and will trigger a refresh once the data expires. This gives requesters a chance to update their data.

test/storage/http_timeout.cpp

@@ -0,0 +1,37 @@
+#include "storage.hpp"
+
+#include <uv.h>
+
+#include <mbgl/storage/default_file_source.hpp>
+#include <mbgl/storage/network_status.hpp>
+
+
+TEST_F(Storage, HTTPTimeout) {
+    SCOPED_TEST(HTTPTimeout)
+
+    using namespace mbgl;
+
+    DefaultFileSource fs(nullptr);
+
+    int counter = 0;
+
+    const Resource resource { Resource::Unknown, "http://127.0.0.1:3000/test?cachecontrol=max-age=1" };
+    Request* req = fs.request(resource, uv_default_loop(), [&](const Response &res) {
+        counter++;
+        EXPECT_EQ(Response::Successful, res.status);
+        EXPECT_EQ(false, res.stale);
+        EXPECT_EQ("Hello World!", res.data);
+        EXPECT_LT(0, res.expires);
+        EXPECT_EQ(0, res.modified);
+        EXPECT_EQ("", res.etag);
+        EXPECT_EQ("", res.message);
+        if (counter == 4) {
+            fs.cancel(req);
+            HTTPTimeout.finish();
+        }
+    });
+
+    uv_run(uv_default_loop(), UV_RUN_DEFAULT);
+
+    EXPECT_EQ(4, counter);
+}