refactor!: use KVStoreService in x/consensus

[facundomedica]

Description

A small PR progressing towards Core API. Here instead of passing the StoreKey we take a KVStoreService

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[facundomedica]

Should I also return the errors instead of panicking? This would change baseapp's ParamStore interface. Then we can just panic when calling StoreConsensusParams if an error is returned

[tac0turtle]

LOVE IT

[julienrbrt]

lgtm, we need a changelog and an UPGRADING.md section for explaining what to change for people not using app wiring.