go/worker/storage: Remove separate storage sync status store

[kostko]

Previously the worker maintaned a separate store that kept information
about the progress of storage sync. Since it was a separate store this
could cause problems if it got out of sync (e.g. due to partial manual
copies). This should make the process more robust as there is only one
source of truth.

[codecov]

Codecov Report

Merging #4565 (0d434bf) into master (0de481e) will increase coverage by 0.15%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #4565      +/-   ##
==========================================
+ Coverage   66.96%   67.11%   +0.15%     
==========================================
  Files         424      424              
  Lines       47970    47955      -15     
==========================================
+ Hits        32121    32183      +62     
+ Misses      11910    11842      -68     
+ Partials     3939     3930       -9     

Impacted Files	Coverage Δ
go/oasis-node/cmd/node/node.go	56.00% <ø> (+1.63%)	⬆️
go/storage/mkvs/db/api/api.go	53.06% <0.00%> (ø)
go/worker/storage/worker.go	88.99% <50.00%> (+1.05%)	⬆️
go/worker/storage/committee/node.go	77.57% <96.42%> (+1.44%)	⬆️
go/consensus/tendermint/abci/prune.go	83.03% <100.00%> (+1.45%)	⬆️
go/consensus/tendermint/abci/state.go	75.36% <100.00%> (+1.60%)	⬆️
go/storage/mkvs/checkpoint/checkpointer.go	83.51% <100.00%> (+0.90%)	⬆️
go/storage/mkvs/db/badger/badger.go	70.28% <100.00%> (-0.05%)	⬇️
go/worker/client/service.go	70.58% <100.00%> (-0.38%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
... and 30 more

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