go consensus: add last retained block information

[pro-wh]

want this for oasisprotocol/oasis-rosetta-gateway#57

[abukosek]

Should this change also bump the ConsensusProtocol version in go/common/version/version.go?

[kostko]

Should this change also bump the ConsensusProtocol version in go/common/version/version.go?

Nope, it's not a breaking consensus change, just adding some fields to the status response.

Issue with error handling.

[codecov]

Codecov Report

Merging #3348 into master will increase coverage by 0.14%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #3348      +/-   ##
==========================================
+ Coverage   65.32%   65.47%   +0.14%     
==========================================
  Files         371      371              
  Lines       33174    33186      +12     
==========================================
+ Hits        21671    21727      +56     
+ Misses       8284     8264      -20     
+ Partials     3219     3195      -24     

Impacted Files	Coverage Δ
go/consensus/api/api.go	14.28% <ø> (ø)
go/consensus/tendermint/full/full.go	61.08% <81.81%> (+0.43%)	⬆️
go/consensus/tests/tester.go	97.95% <100.00%> (+0.02%)	⬆️
go/common/grpc/errors.go	72.34% <0.00%> (-12.77%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	71.58% <0.00%> (-4.80%)	⬇️
go/keymanager/client/client.go	81.41% <0.00%> (-1.77%)	⬇️
go/staking/api/grpc.go	57.98% <0.00%> (-0.85%)	⬇️
go/storage/api/grpc.go	68.28% <0.00%> (ø)
go/storage/mkvs/insert.go	90.41% <0.00%> (ø)
... and 19 more

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