[Execution node] Trie pruning (backport to master)

[ramtinms]

Backport of PR #1507 to master

[codecov-commenter]

Codecov Report

Merging #1578 (dc52b74) into master (c00b407) will increase coverage by 0.04%.
The diff coverage is 86.20%.

@@            Coverage Diff             @@
##           master    #1578      +/-   ##
==========================================
+ Coverage   54.98%   55.03%   +0.04%     
==========================================
  Files         535      535              
  Lines       33069    33109      +40     
==========================================
+ Hits        18184    18221      +37     
  Misses      12452    12452              
- Partials     2433     2436       +3     

Flag	Coverage Δ
unittests	55.03% <86.20%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
ledger/complete/mtrie/node/node.go	68.00% <77.77%> (+7.36%)	⬆️
ledger/complete/ledger.go	61.30% <100.00%> (+0.23%)	⬆️
ledger/complete/mtrie/flattener/forest.go	67.70% <100.00%> (+1.04%)	⬆️
ledger/complete/mtrie/forest.go	58.39% <100.00%> (+0.61%)	⬆️
ledger/complete/mtrie/trie/trie.go	49.50% <100.00%> (+0.76%)	⬆️
admin/command_runner.go	77.58% <0.00%> (ø)

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