Trivial: Replace a global map with a simple function

[codecov-io]

Codecov Report

Merging #882 into master will decrease coverage by 0.23%. The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #882      +/-   ##
==========================================
- Coverage   60.49%   60.26%   -0.24%     
==========================================
  Files         153      153              
  Lines       10708    10713       +5     
==========================================
- Hits         6478     6456      -22     
- Misses       3498     3523      +25     
- Partials      732      734       +2

Flag	Coverage Δ
#integration_tests_long_term	44.31% <50%> (-0.24%)	:arrow_down:
#integration_tests_node	40.38% <50%> (+0.01%)	:arrow_up:
#unittests	48.63% <0%> (-0.03%)	:arrow_down:

Impacted Files	Coverage Δ
lib/transaction/checker.go	81.81% <0%> (+1.46%)	:arrow_up:
lib/transaction/operation/operation.go	40.7% <66.66%> (+1.45%)	:arrow_up:
lib/common/message.go	31.57% <0%> (-10.53%)	:arrow_down:
lib/sync/fetcher.go	57.5% <0%> (-7.5%)	:arrow_down:
lib/sync/try.go	78.57% <0%> (-7.15%)	:arrow_down:
lib/metrics/sync.go	88.57% <0%> (-5.72%)	:arrow_down:
lib/network/validator_connection_manager.go	84.18% <0%> (-3.56%)	:arrow_down:

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