Nominator: Fix combineCandidates introducing new Values

[omerfirmak]

We were serializing the holder instead of ConsensusData

Nominator now also scores the validated values to be used as a nomination criteria for new values to be nominated.

Towards https://github.com/bosagora/agora/issues/3230

[codecov[bot]]

Codecov Report

Merging #3231 (2be5f80) into v0.x.x (f46fdaf) will increase coverage by 1.29%. The diff coverage is 92.68%.

@@            Coverage Diff             @@
##           v0.x.x    #3231      +/-   ##
==========================================
+ Coverage   87.32%   88.61%   +1.29%     
==========================================
  Files         165      166       +1     
  Lines       16966    17072     +106     
==========================================
+ Hits        14816    15129     +313     
+ Misses       2150     1943     -207     

Flag	Coverage Δ
integration	40.34% <62.50%> (?)
unittests	87.33% <92.68%> (+<0.01%)	:arrow_up:

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

Impacted Files	Coverage Δ
source/agora/consensus/protocol/Nominator.d	90.97% <92.68%> (-0.59%)	:arrow_down:
source/agora/consensus/state/ValidatorSet.d	85.71% <0.00%> (-1.06%)	:arrow_down:
source/agora/script/Engine.d	97.59% <0.00%> (-0.16%)	:arrow_down:
source/agora/node/main.d	44.85% <0.00%> (ø)
source/agora/consensus/Ledger.d	97.53% <0.00%> (+0.14%)	:arrow_up:
source/agora/consensus/state/Ledger.d	90.26% <0.00%> (+0.33%)	:arrow_up:
source/agora/node/Validator.d	91.08% <0.00%> (+0.54%)	:arrow_up:
source/agora/network/Manager.d	80.92% <0.00%> (+2.30%)	:arrow_up:
source/agora/node/Registry.d	79.95% <0.00%> (+2.34%)	:arrow_up:
... and 8 more

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[hewison-chris]

I rebased on upstream and pushed as actions had got stuck.