[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.

src/mbgl/map/source.cpp

@@ -149,6 +149,10 @@ void Source::load() {
 
     FileSource* fs = util::ThreadContext::getFileSource();
     req = fs->request({ Resource::Kind::Source, info.url }, util::RunLoop::getLoop(), [this](const Response &res) {
+        if (res.stale) {
+            // Only handle fresh responses.
+            return;
+        }
         req = nullptr;
 
         if (res.status != Response::Successful) {