feat(blobstream): utilizes sdk.Dec type for power difference calculation (backport #2719)

[mergify]

This is an automatic backport of pull request #2719 done by Mergify.
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Changes to be committed:
	modified:   x/qgb/abci.go

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	both modified:   x/qgb/types/types_test.go
	both modified:   x/qgb/types/validator.go

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[evan-forbes]

I'm approving these changes, but am blocking this to make sure we don't accidently merge until we test it on mocha and arabica

[evan-forbes]

to update here, both mocha and arabica are on roughly height 100k with no observed issues yet 🤞

it will likely take a while to finish syncing mocha since it got a ton of spam recently

[evan-forbes]

okay we're gucci