go/storage/api: Handle context cancellation in receiveWriteLogIterator

[kostko]

Exposed by fix for memory leak in #4290, the receiveWriteLogIterator did not handle context cancellation correctly and instead spinned forever.

Also see #4263.

[codecov]

Codecov Report

Merging #4292 (a7719a2) into master (3e4ec06) will decrease coverage by 0.65%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #4292      +/-   ##
==========================================
- Coverage   69.20%   68.54%   -0.66%     
==========================================
  Files         411      411              
  Lines       47215    47217       +2     
==========================================
- Hits        32675    32367     -308     
- Misses      10537    10820     +283     
- Partials     4003     4030      +27     

Impacted Files	Coverage Δ
go/storage/api/grpc.go	67.79% <28.57%> (-1.87%)	⬇️
go/ias/http/http.go	20.33% <0.00%> (-44.07%)	⬇️
go/storage/mkvs/writelog/iterator.go	76.36% <0.00%> (-16.37%)	⬇️
go/keymanager/api/policy_sgx.go	33.33% <0.00%> (-13.34%)	⬇️
go/runtime/host/sandbox/sandbox.go	67.03% <0.00%> (-9.90%)	⬇️
go/common/sgx/aesm/aesm.go	63.63% <0.00%> (-8.19%)	⬇️
go/runtime/host/protocol/connection.go	62.16% <0.00%> (-6.95%)	⬇️
go/oasis-node/cmd/ias/auth.go	68.96% <0.00%> (-6.90%)	⬇️
go/storage/api/mux.go	56.34% <0.00%> (-6.35%)	⬇️
...nsensus/tendermint/apps/beacon/state/state_pvss.go	75.00% <0.00%> (-6.25%)	⬇️
... and 48 more

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