go/worker: Defer various processing

[kostko]

• go/worker/compute/executor: Defer fetching the batch from storage

  There is no need to attempt to fetch the batch immediately, we can defer it to
  when we actually need to start processing the batch. This makes fetching not
  block P2P dispatch.

• go/worker/compute/merge: Defer finalization attempt

  There is no need for the finalization attempt to block handling of an incoming
  commitment as any errors from that are not propagated. This avoids blocking
  P2P relaying as well.

• go/worker/common: Treat unknown P2P message type as a permanent error

[codecov]

Codecov Report

Merging #3049 into master will decrease coverage by 0.08%.
The diff coverage is 81.70%.

@@            Coverage Diff             @@
##           master    #3049      +/-   ##
==========================================
- Coverage   68.46%   68.37%   -0.09%     
==========================================
  Files         370      371       +1     
  Lines       36401    36424      +23     
==========================================
- Hits        24921    24904      -17     
- Misses       8283     8312      +29     
- Partials     3197     3208      +11     

Impacted Files	Coverage Δ
go/worker/common/committee/node.go	72.88% <0.00%> (ø)
go/worker/compute/executor/committee/state.go	88.88% <ø> (+3.70%)	⬆️
go/worker/compute/executor/committee/batch.go	69.23% <69.23%> (ø)
go/worker/compute/merge/committee/node.go	76.38% <77.77%> (-1.51%)	⬇️
go/worker/compute/executor/committee/node.go	64.35% <86.44%> (-2.13%)	⬇️
go/runtime/transaction/batch.go	0.00% <0.00%> (-100.00%)	⬇️
go/storage/mkvs/lookup.go	73.46% <0.00%> (-4.09%)	⬇️
go/storage/metrics.go	75.00% <0.00%> (-3.27%)	⬇️
go/worker/storage/service_external.go	43.01% <0.00%> (-2.16%)	⬇️
... and 22 more

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