go staking: increase reward amount denominator

[pro-wh]

The denominator turned out not to be fine enough. This PR increases it. To keep the same reward rate, the numerators from the genesis block would need to be increased too. cc @bennetyee

[pro-wh]

This new value 100_000_000 is chosen so that the genesis block can specify reward precisely enough to control about a hundredth of a percent APR for rewards that are delivered per-epoch.

[bennetyee]

thanks

[codecov]

Codecov Report

Merging #3065 into master will decrease coverage by 0.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3065      +/-   ##
==========================================
- Coverage   68.31%   67.80%   -0.51%     
==========================================
  Files         372      372              
  Lines       36487    36487              
==========================================
- Hits        24927    24741     -186     
- Misses       8338     8546     +208     
+ Partials     3222     3200      -22     

Impacted Files	Coverage Δ
go/staking/api/rewards.go	60.00% <100.00%> (ø)
go/oasis-node/cmd/ias/auth_registry.go	0.00% <0.00%> (-69.24%)	⬇️
...nsensus/tendermint/apps/keymanager/transactions.go	0.00% <0.00%> (-44.12%)	⬇️
go/keymanager/api/policy_sgx.go	33.33% <0.00%> (-13.34%)	⬇️
go/registry/api/grpc.go	28.74% <0.00%> (-11.32%)	⬇️
go/consensus/tendermint/apps/scheduler/query.go	71.05% <0.00%> (-7.90%)	⬇️
go/consensus/tendermint/apps/staking/query.go	49.09% <0.00%> (-7.28%)	⬇️
...consensus/tendermint/apps/keymanager/keymanager.go	62.92% <0.00%> (-6.75%)	⬇️
go/storage/metrics.go	71.73% <0.00%> (-6.53%)	⬇️
go/staking/api/grpc.go	53.19% <0.00%> (-5.53%)	⬇️
... and 28 more

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