Reduce paddings between fields in stackable classes

[CalvinNeo]

Enhancement

[CalvinNeo]

https://stackoverflow.com/questions/32007188/detect-if-struct-has-padding This is the way to check if a class has padding.

[JaySon-Huang]

A class has padding does not mean it can become a more memory-compact layout.

[CalvinNeo]

@JaySon-Huang Yes you are right, reducing the size of Region by reorg fields is hard.

Size of KVStore can be improved, however, not necessary. The size changed from 1280 to 1216

...
    RegionManager region_manager;

    mutable std::mutex task_mutex;
    mutable std::mutex bg_gc_region_data_mutex;

    // raft_cmd_res stores the result of applying raft cmd. It must be protected by task_mutex.
    std::unique_ptr<RaftCommandResult> raft_cmd_res;
    std::unique_ptr<RegionPersister> region_persister;
    LoggerPtr log;
    ReadIndexWorkerManager * read_index_worker_manager{nullptr};
    const TiFlashRaftProxyHelper * proxy_helper{nullptr};
    std::atomic<UInt64> region_compact_log_min_rows;
    std::atomic<UInt64> region_compact_log_min_bytes;
    std::atomic<UInt64> region_compact_log_gap;
    std::atomic<int64_t> ongoing_prehandle_task_count{0};
    std::atomic<UInt64> region_eager_gc_log_gap;
    std::atomic_int64_t read_index_event_flag{0};
    const bool eager_raft_log_gc_enabled;
    std::atomic<Timepoint> last_gc_time = Timepoint::min();

    std::list<RegionDataReadInfoList> bg_gc_region_data;

    PreHandlingTrace prehandling_trace;
    StoreMeta store;
    RaftLogEagerGcTasks raft_log_gc_hints;
    ProxyConfigSummary proxy_config_summary;

[CalvinNeo]

Alright, the reorg-structure thing is not always make sense, see https://stackoverflow.com/questions/14671253/is-there-a-gcc-keyword-to-allow-structure-reordering/28780286. So arguemnts:

1. Hard to always work correctly
2. May not work well with LTO
3. It is better introduce a [[reorder]] attribute to explicit allows the compiler to reorder layout to have a better layout.