Refactor Epoch

[ShiningRay]

• setup a new Epoch model ( or reues EpochStatistic model )
  • Fields:
    • epoch_number (or id)
    • length
    • start_block_number
    • end_block_number
• refactor the epoch related columns inblocks:
  • use epoch_number ( or epoch_id ) for reference the Epoch model
  • save index_in_epoch for indicating the position of the block in the epoch
  • remove the old epoch, start_number field of blocks
    NOTE: the epoch in block structure on chain is actually a composite data structure

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we could use epoch number for partitioning in blocks table, ckb_transactions table, cell_inputs, cell_outputs table in the future.