Add ability to set initial block height at genesis

[abukosek]

This adds the ability to change the initial block height at genesis time to something other than 1.

[codecov]

Codecov Report

Merging #3416 into master will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3416      +/-   ##
==========================================
- Coverage   66.18%   66.06%   -0.13%     
==========================================
  Files         371      371              
  Lines       33367    33368       +1     
==========================================
- Hits        22085    22045      -40     
- Misses       8041     8097      +56     
+ Partials     3241     3226      -15     

Impacted Files	Coverage Δ
go/oasis-node/cmd/genesis/genesis.go	50.24% <100.00%> (+0.12%)	⬆️
go/worker/compute/executor/committee/state.go	71.42% <0.00%> (-14.29%)	⬇️
go/staking/api/grpc.go	51.26% <0.00%> (-7.57%)	⬇️
go/consensus/tendermint/apps/staking/query.go	35.71% <0.00%> (-4.77%)	⬇️
go/worker/keymanager/handler.go	57.14% <0.00%> (-4.09%)	⬇️
go/worker/compute/executor/committee/node.go	59.09% <0.00%> (-3.93%)	⬇️
go/keymanager/client/client.go	83.18% <0.00%> (-2.66%)	⬇️
go/consensus/tendermint/staking/staking.go	52.83% <0.00%> (-2.52%)	⬇️
go/storage/mkvs/lookup.go	72.63% <0.00%> (-2.11%)	⬇️
go/runtime/committee/nodes.go	69.69% <0.00%> (-1.52%)	⬇️
... and 19 more

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