[core] When reparsing a tile, replace all existing buckets

A tile may go from having particular layers, to not having them.
(Annotation tiles in particular.)

src/mbgl/map/tile_worker.cpp

@@ -287,6 +287,6 @@ void TileWorker::createSymbolBucket(const GeometryTileLayer& layer,
 
 void TileWorker::insertBucket(const std::string& name, std::unique_ptr<Bucket> bucket) {
     if (bucket->hasData()) {
-        result.buckets.emplace_back(name, std::move(bucket));
+        result.buckets.emplace(name, std::move(bucket));
     }
 }

src/mbgl/map/tile_worker.hpp

@@ -31,7 +31,7 @@ class GeometryTileLayer;
 class TileParseResultBuckets {
 public:
     TileData::State state = TileData::State::invalid;
-    std::vector<std::pair<std::string, std::unique_ptr<Bucket>>> buckets;
+    std::unordered_map<std::string, std::unique_ptr<Bucket>> buckets;
 };
 
 using TileParseResult = mapbox::util::variant<TileParseResultBuckets, // success

src/mbgl/map/vector_tile_data.cpp

@@ -39,6 +39,7 @@ VectorTileData::VectorTileData(const TileID& id_,
 
         if (!tile) {
             state = State::parsed;
+            buckets.clear();
             callback();
             return;
         }
@@ -69,9 +70,7 @@ VectorTileData::VectorTileData(const TileID& id_,
 
                 // Move over all buckets we received in this parse request, potentially overwriting
                 // existing buckets in case we got a refresh parse.
-                for (auto& bucket : resultBuckets.buckets) {
-                    buckets[bucket.first] = std::move(bucket.second);
-                }
+                buckets = std::move(resultBuckets.buckets);
 
                 // The target configuration could have changed since we started placement. In this case,
                 // we're starting another placement run.