ADR-040 Implementation: Incorporate v2.MultiStore into BaseApp

[roysc]

Summary

The new MultiStore interface has been merged, but it is not yet used in the app code. The next step of ADR-040 is to integrate it into the BaseApp as the main state storage component.

If we want to maintain seamless backwards compatibility with the v1 MultiStore, this should not form part of a release until a compatibility interface for v1 is also implemented.

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