Ledger: eliminate DB ops in getTxFeeRate()

[omerfirmak]

Which used tp slow down combineCandidates()

[codecov[bot]]

Codecov Report

Merging #3222 (9bf7ceb) into v0.x.x (b8521a6) will increase coverage by 33.91%. The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           v0.x.x    #3222       +/-   ##
===========================================
+ Coverage   54.74%   88.66%   +33.91%     
===========================================
  Files         331      167      -164     
  Lines       35753    17038    -18715     
===========================================
- Hits        19572    15106     -4466     
+ Misses      16181     1932    -14249     

Flag	Coverage Δ
integration	33.91% <ø> (+3.67%)	:arrow_up:
unittests	87.40% <83.33%> (+0.06%)	:arrow_up:

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

Impacted Files	Coverage Δ
source/agora/consensus/pool/Transaction.d	93.69% <81.81%> (-0.20%)	:arrow_down:
source/agora/consensus/Ledger.d	97.48% <100.00%> (-0.02%)	:arrow_down:
source/agora/common/VibeTask.d	0.00% <0.00%> (-86.67%)	:arrow_down:
source/agora/common/FileBasedLock.d	0.00% <0.00%> (-72.73%)	:arrow_down:
source/agora/consensus/BlockStorage.d	68.90% <0.00%> (-11.31%)	:arrow_down:
source/agora/consensus/state/ValidatorSet.d	85.71% <0.00%> (-1.06%)	:arrow_down:
source/agora/common/DNS.d	87.96% <0.00%> (-0.40%)	:arrow_down:
source/agora/consensus/EnrollmentManager.d	96.97% <0.00%> (-0.31%)	:arrow_down:
agora/submodules/vibe.d/data/vibe/data/json.d
...a/submodules/vibe.d/data/vibe/data/serialization.d
... and 169 more

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[omerfirmak]

oom?