go: Various requested gRPC API additions.

[Yawning]

• Expose list of delegators for given height via gRPC #2740 Expose list of delegators for given height via gRPC
• Expose NodeByConsensusAddress via gRPC #2739 Expose NodeByConsensusAddress via gRPC

Notes:

• The new Delegations call mimics the existing DebondingDelegations call, and requires specifying the delegator ID.

[codecov]

Codecov Report

Merging #2750 into master will decrease coverage by 0.01%.
The diff coverage is 26.47%.

@@            Coverage Diff             @@
##           master    #2750      +/-   ##
==========================================
- Coverage   62.77%   62.75%   -0.02%     
==========================================
  Files         383      383              
  Lines       36097    36159      +62     
==========================================
+ Hits        22659    22691      +32     
- Misses      10594    10597       +3     
- Partials     2844     2871      +27

Impacted Files	Coverage Δ
go/scheduler/api/api.go	71.15% <ø> (ø)	⬆️
go/staking/api/api.go	71.42% <ø> (ø)	⬆️
go/scheduler/tests/tester.go	94.24% <ø> (ø)	⬆️
...o/consensus/tendermint/apps/staking/state/state.go	62.76% <0%> (-2.66%)	⬇️
go/consensus/tendermint/staking/staking.go	68% <0%> (-2.35%)	⬇️
go/staking/api/grpc.go	53.33% <0%> (-3.81%)	⬇️
go/consensus/tendermint/apps/staking/query.go	64.86% <0%> (-3.71%)	⬇️
go/consensus/tendermint/apps/registry/query.go	76.36% <100%> (+3.44%)	⬆️
go/consensus/tendermint/apps/scheduler/query.go	63.63% <43.75%> (-13.04%)	⬇️
go/common/grpc/proxy/proxy.go	58.87% <0%> (-5.61%)	⬇️
... and 14 more

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[peterjgilbert]

LGTM