Page History

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1. acquire page
   1. if checkpoint is finished (after key change) and page is dirty - skip this page.
   2. if checkpoint is not finished or page is not dirty
      1. lock page
      2. if checkpoint is not finished and page is dirty - save additional page snapshot into WALt
      3. unlock page (dirty=true, if page hasn't been dirty snapshot is logged into WAL)
2. release page

Re-encryption progress is stored into metapage (int offset, int total), updates during checkpoint.

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