Method fetch_current_handler

src/runtime/task/task.h:504–504

rpc_response_task is a special kind of task, because we support the semantic of distributed service in sending rpc request, for example: "querying meta-server and send message to proper replica-server based on gpid". in order to support this, we need to replace the original rpc response callback to another one: 1. which can proecss the distributed service semantics, AND 2. which won't call the or

    // TODO(sunweijie): totally elimite this feature
    //
    void fetch_current_handler(rpc_response_handler &cb) { cb = std::move(_cb); }
    void replace_callback(rpc_response_handler &&cb)
    {
        task_state cur_state = state();