[SCN REFACTOR] replace flash back snapshot and remove read snapshot session

src/share/allocator/ob_reserve_arena.h

@@ -80,15 +80,19 @@ public:
   }
   virtual void reset() override
   {
+    if (pos_ > MAX_RESERVE_SIZE - 1) {
+      allocator_.reset();
+    }
     pos_ = 0;
     buf_[0] = '\0';
-    allocator_.reset();
   }
   virtual void reuse() override
   {
+	  if (pos_ > MAX_RESERVE_SIZE - 1) {
+      allocator_.reuse();
+    }
     pos_ = 0;
     buf_[0] = '\0';
-    allocator_.reuse();
   }
 
   virtual void set_attr(const ObMemAttr &attr) { UNUSED(attr); }