Move history of contract code migrations to their own prefix store.

[alpe]

Summary

Move history of contract code migrations to their own prefix store. This would reduce the overhead in de-/serialization data for ContractInfo and save gas.

Follow up on #130

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[ethanfrey]

Good idea.

I think they can have a different prefix store, but one element per contract. Using the same address as key.

Definitely makes most queries much cheaper when there is much migration

[alpe]

👍 Another benefit is easier genesis ex-/ import as there is no need to import the history