[MPoW] Merged work report

[badkk]

1. 2 id strategy
2. Merge files strategy
3. Merge reserved strategy:
   • Adding cached_reserved for caching the A/B work report(with FIXME bug);
   • Change reported_in_slot to (elder_reported, current_reported) to avoid double storage during upgrading;
4. Change test cases:
   • Change error code: https://github.com/crustio/crust/compare/zikun/merged-wr?expand=1#diff-2766953e0bb65080609e9ae353b1d4b6L554
   • Change all data structure(not break the test logic);