[core] Don't allow style mutations to be overwritten by revalidation

* Once we get a fresh style, stop revalidating.
* If the style is mutated, stop revalidating and preserve the existing mutations.

include/mbgl/storage/response.hpp

@@ -32,6 +32,10 @@ public:
     optional<Timestamp> modified;
     optional<Timestamp> expires;
     optional<std::string> etag;
+
+    bool isFresh() const {
+        return !expires || *expires > util::now();
+    }
 };
 
 class Response::Error {