Nominator: Avoid nominating inferior values

[omerfirmak]

this will help minimize the number of values network is voting on

Fixes #3230

[hewison-chris]

Rebased on upstream.

[codecov[bot]]

Codecov Report

Merging #3234 (3120906) into v0.x.x (22ad709) will decrease coverage by 33.72%. The diff coverage is 87.50%.

@@             Coverage Diff             @@
##           v0.x.x    #3234       +/-   ##
===========================================
- Coverage   88.36%   54.64%   -33.73%     
===========================================
  Files         166      330      +164     
  Lines       17065    35783    +18718     
===========================================
+ Hits        15080    19553     +4473     
- Misses       1985    16230    +14245     

Flag	Coverage Δ
integration	29.41% <87.50%> (-4.94%)	:arrow_down:
unittests	87.41% <87.50%> (+0.04%)	: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	91.21% <87.50%> (+0.26%)	:arrow_up:
source/agora/node/Validator.d	91.95% <0.00%> (-1.01%)	:arrow_down:
source/agora/consensus/state/Ledger.d	90.60% <0.00%> (-0.34%)	:arrow_down:
source/agora/consensus/Ledger.d	97.38% <0.00%> (-0.15%)	:arrow_down:
source/agora/network/Manager.d	80.59% <0.00%> (ø)
agora/submodules/vibe.d/data/vibe/data/bson.d	0.76% <0.00%> (ø)
...s/eventcore/source/eventcore/drivers/posix/epoll.d	66.66% <0.00%> (ø)
agora/submodules/diet-ng/source/diet/dom.d	0.00% <0.00%> (ø)
agora/submodules/vibe.d/inet/vibe/inet/message.d	0.70% <0.00%> (ø)
agora/submodules/configy/source/configy/Utils.d	0.00% <0.00%> (ø)
... and 168 more

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