go/worker/common: Move committee storage to Group

[kostko]

Move committee storage to Group to make it available to all subworkers (instead of the executor worker having a specialized case) and change it so that it doesn't go through gRPC when communicating with the local node.

[codecov]

Codecov Report

Merging #4061 (968eeb0) into master (30a66c9) will decrease coverage by 0.89%.
The diff coverage is 79.68%.

@@            Coverage Diff             @@
##           master    #4061      +/-   ##
==========================================
- Coverage   67.30%   66.41%   -0.90%     
==========================================
  Files         411      411              
  Lines       42162    42190      +28     
==========================================
- Hits        28379    28022     -357     
- Misses       9828    10190     +362     
- Partials     3955     3978      +23     

Impacted Files	Coverage Δ
go/oasis-node/cmd/node/node.go	54.56% <0.00%> (-1.73%)	⬇️
go/worker/common/committee/node.go	74.03% <0.00%> (-0.72%)	⬇️
go/runtime/registry/registry.go	76.77% <50.00%> (+0.09%)	⬆️
go/storage/client/client.go	75.00% <81.25%> (-1.55%)	⬇️
go/worker/common/committee/group.go	80.60% <85.71%> (+0.88%)	⬆️
go/storage/client/init.go	75.60% <100.00%> (+0.60%)	⬆️
go/worker/compute/executor/committee/node.go	68.11% <100.00%> (-1.29%)	⬇️
go/oasis-node/cmd/ias/auth_registry.go	0.00% <0.00%> (-67.13%)	⬇️
...nsensus/tendermint/apps/keymanager/transactions.go	0.00% <0.00%> (-52.95%)	⬇️
go/ias/http/http.go	18.51% <0.00%> (-44.45%)	⬇️
... and 56 more

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