[BACKPORT/21.2.x] go/storage/mkvs/checkpoint: Determine next offset correctly

[kostko]

Backport of #4096

[codecov]

Codecov Report

Merging #4097 (645c60f) into stable/21.2.x (f88cdf9) will decrease coverage by 0.77%.
The diff coverage is 67.78%.

@@                Coverage Diff                @@
##           stable/21.2.x    #4097      +/-   ##
=================================================
- Coverage          67.19%   66.42%   -0.78%     
=================================================
  Files                414      414              
  Lines              42329    42455     +126     
=================================================
- Hits               28445    28200     -245     
- Misses              9908    10273     +365     
- Partials            3976     3982       +6     

Impacted Files	Coverage Δ
go/consensus/api/api.go	14.28% <ø> (ø)
go/consensus/tendermint/api/state.go	60.27% <0.00%> (+1.94%)	⬆️
go/consensus/tendermint/seed/seed.go	67.33% <0.00%> (-0.46%)	⬇️
go/ias/proxy/client/client.go	58.46% <0.00%> (-1.86%)	⬇️
go/storage/api/api.go	70.37% <ø> (ø)
go/worker/compute/executor/committee/node.go	67.54% <23.07%> (-1.97%)	⬇️
go/runtime/registry/registry.go	74.90% <37.50%> (-1.87%)	⬇️
go/consensus/tendermint/full/full.go	62.67% <55.55%> (-2.69%)	⬇️
go/runtime/nodes/runtime.go	75.53% <60.00%> (-3.22%)	⬇️
go/worker/storage/committee/node.go	77.76% <61.81%> (-1.50%)	⬇️
... and 76 more

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