go/extra/stats: Update availability formula

[peterjgilbert]

As per https://docs.oasis.dev/operators/the-quest-rules.html#types-of-challenges, the availability score formula has changed from Blocks Signed + 50 x Blocks Proposed to Blocks Signed + 50 x Blocks Proposed in Round 0.

[codecov]

Codecov Report

Merging #2819 into master will increase coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2819      +/-   ##
==========================================
+ Coverage   65.77%   66.00%   +0.23%     
==========================================
  Files         342      342              
  Lines       32579    32579              
==========================================
+ Hits        21428    21503      +75     
+ Misses       8448     8389      -59     
+ Partials     2703     2687      -16     

Impacted Files	Coverage Δ
go/common/cbor/codec.go	64.70% <0.00%> (-5.89%)	⬇️
go/common/grpc/auth/auth.go	88.88% <0.00%> (-5.56%)	⬇️
go/storage/api/grpc.go	69.89% <0.00%> (-0.67%)	⬇️
go/roothash/tests/tester.go	95.54% <0.00%> (-0.64%)	⬇️
go/worker/keymanager/worker.go	59.40% <0.00%> (-0.60%)	⬇️
go/common/grpc/grpc.go	78.42% <0.00%> (-0.42%)	⬇️
go/consensus/tendermint/tendermint.go	68.11% <0.00%> (+0.27%)	⬆️
go/storage/mkvs/db/badger/badger.go	52.91% <0.00%> (+0.44%)	⬆️
go/runtime/client/client.go	68.26% <0.00%> (+0.73%)	⬆️
go/worker/compute/merge/committee/node.go	76.61% <0.00%> (+0.99%)	⬆️
... and 15 more

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[pro-wh]

ok

now that we need to look at the commit info for each block, we have to look one block ahead, when the commit info is agreed upon. and when we looking one block ahead, we have to remember the proposer for the block we're evaluating the score for