MKVS fixes and improvements

[kostko]

Extracted from #2674 for easier review.

Contains a fix for an MKVS iterator bug.

[codecov]

Codecov Report

Merging #2691 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2691      +/-   ##
==========================================
- Coverage   63.09%   63.03%   -0.07%     
==========================================
  Files         376      377       +1     
  Lines       35325    35440     +115     
==========================================
+ Hits        22289    22339      +50     
- Misses      10236    10289      +53     
- Partials     2800     2812      +12     

Impacted Files	Coverage Δ
go/consensus/tendermint/api/api.go	73.58% <0.00%> (-15.10%)	⬇️
go/consensus/api/grpc.go	57.02% <0.00%> (-6.62%)	⬇️
go/runtime/committee/nodes.go	80.64% <0.00%> (-2.16%)	⬇️
go/consensus/tendermint/tendermint.go	66.48% <0.00%> (-1.67%)	⬇️
...o/consensus/tendermint/apps/scheduler/scheduler.go	71.70% <0.00%> (-1.38%)	⬇️
go/runtime/transaction/transaction.go	75.90% <0.00%> (-1.21%)	⬇️
go/worker/compute/executor/committee/node.go	60.82% <0.00%> (-1.19%)	⬇️
go/storage/client/client.go	77.31% <0.00%> (-0.85%)	⬇️
go/worker/compute/merge/committee/node.go	75.37% <0.00%> (-0.50%)	⬇️
go/storage/mkvs/urkel/overlay.go	73.52% <0.00%> (ø)
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1a68809...91efcae. Read the comment docs.