Improve mempool extraction

[herr-seppia]

Resolves #1421

[codecov-commenter]

Codecov Report

Merging #1422 (1b1ef4c) into master (828364b) will decrease coverage by 9.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1422      +/-   ##
==========================================
- Coverage   50.51%   41.03%   -9.48%     
==========================================
  Files         145      144       -1     
  Lines        7544     9467    +1923     
==========================================
+ Hits         3811     3885      +74     
- Misses       3066     4950    +1884     
+ Partials      667      632      -35     

Impacted Files	Coverage Δ
pkg/core/consensus/testing/node.go	0.00% <0.00%> (-93.55%)	⬇️
...ore/consensus/blockgenerator/candidate/testutil.go	0.00% <0.00%> (-90.91%)	⬇️
pkg/core/consensus/testing/utils.go	0.00% <0.00%> (-75.61%)	⬇️
...core/consensus/blockgenerator/candidate/genesis.go	0.00% <0.00%> (-69.24%)	⬇️
...nsensus/blockgenerator/candidate/blockgenerator.go	0.00% <0.00%> (-64.29%)	⬇️
pkg/gql/query/transactions.go	13.43% <0.00%> (-52.55%)	⬇️
pkg/rpc/client/session.go	21.42% <0.00%> (-47.15%)	⬇️
pkg/util/nativeutils/eventbus/eventbus.go	57.14% <0.00%> (-42.86%)	⬇️
pkg/rpc/client/client.go	3.73% <0.00%> (-36.61%)	⬇️
pkg/core/consensus/reduction/aggregator.go	56.25% <0.00%> (-35.06%)	⬇️
... and 166 more

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[t00ts]

LGTM, with couple nits.