Separate Connection and Request lifetimes

src/arena.hpp

@@ -406,6 +406,7 @@ public:
    * ## Note:
    * This method only allocates memory - it does not construct objects.
    * Use placement new or other initialization methods as needed.
+   * TODO should this return a std::span<T> ?
    */
   template <typename T> T *allocate(uint32_t size) {
     static_assert(