go/extra/stats: fix heights at which missing nodes should be queried

[ptrus]

If we encounter a signature for a node that is not yet being tracked, we should query for it's info on the previous height, as that's the block at which the vote was made.

Tested on blocks 0-69488 questnet.

[codecov]

Codecov Report

Merging #2858 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2858      +/-   ##
==========================================
- Coverage   67.82%   67.70%   -0.12%     
==========================================
  Files         344      344              
  Lines       33426    33426              
==========================================
- Hits        22670    22631      -39     
- Misses       7869     7897      +28     
- Partials     2887     2898      +11     

Impacted Files	Coverage Δ
go/storage/metrics.go	88.04% <0.00%> (-3.27%)	⬇️
go/consensus/tendermint/roothash/roothash.go	65.54% <0.00%> (-2.37%)	⬇️
go/storage/api/root_cache.go	77.01% <0.00%> (-2.30%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	55.43% <0.00%> (-2.18%)	⬇️
go/consensus/tendermint/abci/mux.go	69.14% <0.00%> (-2.03%)	⬇️
...consensus/tendermint/apps/scheduler/state/state.go	71.15% <0.00%> (-1.93%)	⬇️
go/worker/compute/txnscheduler/committee/node.go	64.50% <0.00%> (-1.86%)	⬇️
go/storage/database/database.go	79.09% <0.00%> (-1.82%)	⬇️
go/consensus/tendermint/apps/staking/fees.go	38.39% <0.00%> (-1.79%)	⬇️
go/roothash/api/commitment/pool.go	69.94% <0.00%> (-1.69%)	⬇️
... and 18 more

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