[BACKPORT] go/extra/stats: fix & simplify node-entity mapping

[ptrus]

Backports: #2856

Instead of separately querying for entities and nodes, we can get Entity IDs
from nodes directly.

This change also fixes a case that previous variant missed: node that was
removed from entity list of nodes, but has not yet expired.

[codecov]

Codecov Report

Merging #2857 into stable/20.5.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           stable/20.5.x    #2857      +/-   ##
=================================================
- Coverage          68.30%   68.24%   -0.07%     
=================================================
  Files                342      342              
  Lines              33094    33094              
=================================================
- Hits               22606    22586      -20     
- Misses              7758     7762       +4     
- Partials            2730     2746      +16     

Impacted Files	Coverage Δ
go/consensus/tendermint/api/api.go	73.58% <0.00%> (-15.10%)	⬇️
go/storage/api/root_cache.go	72.41% <0.00%> (-4.60%)	⬇️
go/storage/mkvs/tree.go	89.28% <0.00%> (-3.58%)	⬇️
...o/consensus/tendermint/apps/scheduler/scheduler.go	71.70% <0.00%> (-3.03%)	⬇️
go/storage/mkvs/insert.go	87.75% <0.00%> (-2.73%)	⬇️
go/storage/client/client.go	75.33% <0.00%> (-2.65%)	⬇️
go/worker/common/committee/group.go	79.01% <0.00%> (-1.97%)	⬇️
go/storage/database/database.go	77.27% <0.00%> (-1.82%)	⬇️
go/worker/compute/merge/committee/node.go	75.87% <0.00%> (-1.75%)	⬇️
go/storage/mkvs/cache.go	81.27% <0.00%> (-1.71%)	⬇️
... and 12 more

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