go/storage/mkvs: Add MKVS fuzzing

[kostko]

Adds fuzzers specifically for MKVS tree operations and fixes a bug found during fuzzing (added as unit test).

[codecov]

Codecov Report

Merging #2698 into master will decrease coverage by 0.06%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master    #2698      +/-   ##
==========================================
- Coverage    63.2%   63.14%   -0.07%     
==========================================
  Files         377      377              
  Lines       35440    35448       +8     
==========================================
- Hits        22400    22382      -18     
- Misses      10242    10261      +19     
- Partials     2798     2805       +7

Impacted Files	Coverage Δ
go/storage/mkvs/urkel/insert.go	86.39% <0%> (-3.96%)	⬇️
go/storage/mkvs/urkel/node/key.go	77.41% <100%> (+0.75%)	⬆️
go/storage/client/tests/tests.go	90.47% <100%> (+0.15%)	⬆️
go/storage/mkvs/urkel/remove.go	85.41% <100%> (+0.31%)	⬆️
go/worker/compute/executor/committee/state.go	74.07% <0%> (-11.12%)	⬇️
go/storage/mkvs/urkel/urkel.go	85.71% <0%> (-3.58%)	⬇️
go/worker/compute/executor/committee/node.go	62% <0%> (-3.35%)	⬇️
go/storage/metrics.go	75.21% <0%> (-2.57%)	⬇️
go/storage/api/root_cache.go	72.41% <0%> (-2.3%)	⬇️
... and 14 more

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