refactor(x/distribution)!: Use KVStoreService, context.Context and return errors instead of panic

[facundomedica]

Description

Here we:

• Move from storeKey to KVStoreService
• Change usages of sdk.Context to context.Context (as many as possible, some are still needed)
• Return errors instead of panic whenever possible

Notes for reviewers:

• Not all errors are currently possible to return instead of panicking (for example when iterating through validators), so we'll need follow up PRs
• We need to make sure we are not ignoring errors. Functions that were previously panicking are now returning errors

TODO for follow up PRs:

• Convert any panic("hardcoded string") into return types.ErrSomethingSpecific

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[julienrbrt]

lgtm!