go/worker/executor: don't hold the lock during storage requests

[ptrus]

Don't unnecessarily hold the lock during external requests to storage nodes, as those can take long time.

Issue discovered in the daily tests with long restarts, when the storage node is taken offline, these requests can take up to 15 sec to timeout.

[codecov]

Codecov Report

Merging #3320 into master will increase coverage by 0.51%.
The diff coverage is 55.00%.

@@            Coverage Diff             @@
##           master    #3320      +/-   ##
==========================================
+ Coverage   64.89%   65.40%   +0.51%     
==========================================
  Files         371      371              
  Lines       33045    33062      +17     
==========================================
+ Hits        21443    21623     +180     
+ Misses       8404     8267     -137     
+ Partials     3198     3172      -26     

Impacted Files	Coverage Δ
go/worker/compute/executor/committee/node.go	64.10% <55.00%> (+3.25%)	⬆️
go/common/grpc/errors.go	79.06% <0.00%> (-9.31%)	⬇️
go/worker/common/p2p/dispatch.go	74.52% <0.00%> (-4.72%)	⬇️
...n/crypto/signature/signers/memory/memory_signer.go	72.41% <0.00%> (-3.45%)	⬇️
go/keymanager/client/client.go	81.41% <0.00%> (-1.77%)	⬇️
go/worker/common/committee/node.go	74.86% <0.00%> (ø)
go/consensus/tendermint/roothash/roothash.go	70.76% <0.00%> (ø)
go/worker/storage/committee/checkpoint_sync.go	73.38% <0.00%> (ø)
go/runtime/host/protocol/connection.go	57.20% <0.00%> (+0.82%)	⬆️
go/consensus/tendermint/full/full.go	60.58% <0.00%> (+0.92%)	⬆️
... and 26 more

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