Use shared pointers for transaction attempt object

Callback-based (bulk or async) API cannot rely on C++ references, because it is executed asynchronously.

We might want to allow the pointer to attempt object to survive the logic lambda, so that the following code will be possible

txn.run([](auto attempt)  {

    attempt.get([attempt] (auto ctx, auto res) {

      if(ctx.ec()) {

        // handle error

      } else {

        attempt.replace(...)

      }

    });

});