feat(lib/grandpa): improve determinePrevote method

[EclesioMeloJunior]

Changes

• Ensure the block we vote on belongs to the same chain as latestFinalizedHeader
• Don't include the BeforeBestBlockBy voting rule as I did not find too much information about it (probably include in other PR) refs
  • https://github.com/paritytech/polkadot/blob/270fb1e576ae2199c8b1803ec91a9a749ded6779/node/service/src/lib.rs#L1250
  • https://github.com/paritytech/substrate/blob/20b5aac7e09e8b25abaa48c6781e653b36ba3ca0/client/finality-grandpa/src/voting_rule.rs#L92

Tests

N/A

Issues

• Related to PR fix(lib/grandpa): avoid spamming round messages #2688
• Related to Polite GRANDPA #2505

Primary Reviewer

@timwu20

[codecov]

Codecov Report

Merging #2745 (33a12a8) into development (426569a) will increase coverage by 0.09%.
The diff coverage is 53.57%.

Additional details and impacted files

@@               Coverage Diff               @@
##           development    #2745      +/-   ##
===============================================
+ Coverage        63.12%   63.21%   +0.09%     
===============================================
  Files              213      213              
  Lines            27034    26991      -43     
===============================================
- Hits             17064    17063       -1     
+ Misses            8420     8375      -45     
- Partials          1550     1553       +3