Consider removing storage_occ_* from settings

[BohuTANG]

*************************** 9. row ***************************
       name: storage_occ_backoff_init_delay_ms
      value: 5
    default: 5
      level: SESSION
description: The initial retry delay in millisecond. By default, it is 5 ms.
       type: UInt64
*************************** 10. row ***************************
       name: storage_occ_backoff_max_delay_ms
      value: 20000
    default: 20000
      level: SESSION
description: The maximum  back off delay in millisecond, once the retry interval reaches this value, it stops increasing. By default, it is 20 seconds.
       type: UInt64
*************************** 11. row ***************************
       name: storage_occ_backoff_max_elapsed_ms
      value: 120000
    default: 120000
      level: SESSION
description: The maximum elapsed time after the occ starts, beyond which there will be no more retries. By default, it is 2 minutes.
       type: UInt64
*************************** 12. row ***************************

We should treat storage_occ_* as the internal settings, make user won't care about them.
We can default value for them first, if the users need setting them by mannual someday, add them back to system.settings.

[BohuTANG]

@dantengsky any comments on it?

[dantengsky]

sounds reasonable to me. Never tuned them in practice yet