librbd: notify watcher when updating image metadata

librbd: notify watcher when updating image metadata

Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
(cherry picked from commit b10d26dfa84627b2622d405d272b1133bb773245)

Conflicts:
    src/librbd/image/RefreshRequest.h (luminous does not have
        233482cf708aa7b4723c529d387c4605bdf0722f so we leave the following variables
        uninitialized:
          uint8_t m_order;
          uint64_t m_size;
          uint64_t m_features;
          uint64_t m_incompatible_features;
          uint64_t m_flags;
        )