Optimize Ledger.Get() by making Forest.Read() use ~5x fewer allocs/op and run faster

[fxamacker]

Changes

Change Forest.Read() to return []ledger.Value without deep copying payload keys. This avoids 4 heap allocation per key.

This change doesn't affect Ledger.Get() (the caller) because it discards the payload keys.

Benchmark comparison for batch read size of 100 keys

name        old time/op    new time/op    delta
TrieRead100-4     524µs ± 1%     420µs ± 1%  -19.77%  (p=0.000 n=10+10)

name        old alloc/op   new alloc/op   delta
TrieRead100-4     190kB ± 0%      95kB ± 0%  -50.04%  (p=0.000 n=10+10)

name        old allocs/op  new allocs/op  delta
TrieRead100-4     1.52k ± 0%     0.32k ± 0%  -79.17%  (p=0.000 n=10+10)

Closes #2475

[codecov-commenter]

Codecov Report

Merging #2476 (f026cc3) into master (1d6be74) will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #2476      +/-   ##
==========================================
+ Coverage   56.69%   56.72%   +0.03%     
==========================================
  Files         656      656              
  Lines       60175    60217      +42     
==========================================
+ Hits        34114    34157      +43     
+ Misses      23123    23122       -1     
  Partials     2938     2938              

Flag	Coverage Δ
unittests	56.72% <83.33%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
ledger/complete/mtrie/forest.go	62.96% <80.00%> (ø)
ledger/complete/ledger.go	60.77% <100.00%> (+0.45%)	⬆️
...s/hotstuff/votecollector/staking_vote_processor.go	83.87% <0.00%> (-3.23%)	⬇️
insecure/wintermute/helpers.go	96.64% <0.00%> (+0.07%)	⬆️
engine/execution/ingestion/engine.go	52.54% <0.00%> (+0.28%)	⬆️
insecure/wintermute/attackOrchestrator.go	72.39% <0.00%> (+0.50%)	⬆️
engine/verification/verifier/engine.go	51.55% <0.00%> (+0.76%)	⬆️
insecure/corruptible/factory.go	62.80% <0.00%> (+4.93%)	⬆️
module/mempool/epochs/transactions.go	100.00% <0.00%> (+9.67%)	⬆️

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[fxamacker]

Closing this because it was based on master. I'll open new PR based on PR #2473.