staking/state: fix DelegationsFor queries

[ptrus]

Split out from: #2752 (unit test will be added there)

cc @peterjgilbert

[codecov]

Codecov Report

Merging #2756 into master will decrease coverage by 0.15%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master    #2756      +/-   ##
==========================================
- Coverage   62.89%   62.74%   -0.16%     
==========================================
  Files         384      384              
  Lines       36253    36257       +4     
==========================================
- Hits        22802    22749      -53     
- Misses      10560    10617      +57     
  Partials     2891     2891              

Impacted Files	Coverage Δ
...o/consensus/tendermint/apps/staking/state/state.go	62.17% <16.66%> (-0.60%)	⬇️
go/storage/metrics.go	72.64% <0.00%> (-10.26%)	⬇️
go/storage/api/root_cache.go	70.11% <0.00%> (-6.90%)	⬇️
...ompute/txnscheduler/algorithm/batching/batching.go	78.66% <0.00%> (-6.67%)	⬇️
go/storage/database/database.go	74.56% <0.00%> (-5.27%)	⬇️
go/consensus/api/grpc.go	59.30% <0.00%> (-4.66%)	⬇️
go/storage/mkvs/urkel/urkel.go	85.71% <0.00%> (-3.58%)	⬇️
go/oasis-node/cmd/debug/storage/storage.go	45.08% <0.00%> (-3.47%)	⬇️
go/storage/mkvs/urkel/insert.go	86.39% <0.00%> (-2.73%)	⬇️
go/consensus/tendermint/roothash/roothash.go	68.91% <0.00%> (-2.63%)	⬇️
... and 13 more

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[pro-wh]

ah of course, these aren't stored contiguously