planning: prefer unique_ptr than shared_ptr

modules/planning/reference_line/reference_line_provider.cc

@@ -106,9 +106,9 @@ bool ReferenceLineProvider::Start() {
   }
 
   if (FLAGS_enable_reference_line_provider_thread) {
-    task_ = std::make_shared<apollo::cybertron::Task<int>>(
+    task_.reset(new apollo::cybertron::Task<int>(
         "async_reference_line_provider",
-        [this](const std::shared_ptr<int> &) { this->GenerateThread(); });
+        [this](const std::shared_ptr<int> &) { this->GenerateThread(); }));
     task_future_ = task_->Execute(std::make_shared<int>());
   }
   return true;

modules/planning/reference_line/reference_line_provider.h

@@ -180,7 +180,7 @@ class ReferenceLineProvider {
   std::queue<std::list<ReferenceLine>> reference_line_history_;
   std::queue<std::list<hdmap::RouteSegments>> route_segments_history_;
 
-  std::shared_ptr<cybertron::Task<int>> task_;
+  std::unique_ptr<cybertron::Task<int>> task_;
   std::future<void> task_future_;
 };