[core] reparse tiles when new data arrives

We're now reparsing tiles when they expire. We're also swapping out buckets atomically to avoid flickering data; i.e. we're displaying the old data as long as we don't have a new parsed bucket for that layer yet. The parsed buckets now live in the *TileData objects rather than in the TileWorker; only partially parsed == pending buckets will remain in the TileWorker. Once they're parsed, they're moved to the *TileData object.

include/mbgl/storage/response.hpp

@@ -2,6 +2,7 @@
 #define MBGL_STORAGE_RESPONSE
 
 #include <string>
+#include <memory>
 
 namespace mbgl {