[Protocol State] Combine the caches for last finalized and last sealed block

[zhangchiqing]

Since finalized block and sealed block are updated together, the cache could be update together as well. Combining them could eliminate any edge case when accessing the cache.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 41.08%. Comparing base (8c170e3) to head (357925d).

Files with missing lines	Patch %	Lines
state/protocol/badger/state.go	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6915      +/-   ##
==========================================
- Coverage   41.09%   41.08%   -0.02%     
==========================================
  Files        2121     2121              
  Lines      185912   185914       +2     
==========================================
- Hits        76395    76377      -18     
- Misses     103104   103120      +16     
- Partials     6413     6417       +4     

Flag	Coverage Δ
unittests	41.08% <90.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.