[enhancement] Refactor to improve the usability of MemTracker (step2) (#10823)

be/src/runtime/bufferpool/buffer_allocator.h

@@ -19,7 +19,7 @@
 
 #include "runtime/bufferpool/buffer_pool_internal.h"
 #include "runtime/bufferpool/free_list.h"
-#include "runtime/mem_tracker.h"
+#include "runtime/memory/mem_tracker.h"
 #include "util/aligned_new.h"
 
 namespace doris {
@@ -236,6 +236,6 @@ private:
     /// but is guaranteed to succeed.
     int max_scavenge_attempts_;
 
-    std::shared_ptr<MemTracker> _mem_tracker;
+    std::unique_ptr<MemTracker> _mem_tracker;
 };
 } // namespace doris