Cleanup the MapContext before the util::Thread destructor

The util::Thread will call the stop() method of the MapContext's RunLoop
which will wait for the pending tasks tied to it to complete.

If we have a request that is timeout'ing, this could mean a really long
wait. Instead, we now send a cleanup message that will reset all the
attributes first (canceling the pending requests) so the the MapContext
gets destructed quickly.

src/mbgl/map/map.cpp

@@ -17,6 +17,7 @@ Map::Map(View& view, FileSource& fileSource, MapMode mode)
 
 Map::~Map() {
     resume();
+    context->invoke(&MapContext::cleanup);
 }
 
 void Map::pause() {

src/mbgl/map/map_context.cpp

@@ -36,6 +36,11 @@ MapContext::MapContext(uv_loop_t* loop, View& view_, FileSource& fileSource, Map
 }
 
 MapContext::~MapContext() {
+    // Make sure we call cleanup() before deleting this object.
+    assert(!style);
+}
+
+void MapContext::cleanup() {
     view.notify();
 
     // Explicit resets currently necessary because these abandon resources that need to be

src/mbgl/map/map_context.hpp

@@ -54,6 +54,8 @@ public:
     void setSourceTileCacheSize(size_t size);
     void onLowMemory();
 
+    void cleanup();
+
     // Style::Observer implementation.
     void onTileDataChanged() override;
     void onResourceLoadingFailed(std::exception_ptr error) override;

test/miscellaneous/map_context.cpp

@@ -19,4 +19,5 @@ TEST(MapContext, DoubleStyleLoad) {
 
     context.invokeSync(&MapContext::setStyleJSON, "", "");
     context.invokeSync(&MapContext::setStyleJSON, "", "");
+    context.invokeSync(&MapContext::cleanup);
 }