Problem: new iavl indexes migration is slow and not optional (backport: #714)

[yihuang]

👮🏻👮🏻👮🏻 !!!! REFERENCE THE PROBLEM YOUR ARE SOLVING IN THE PR TITLE AND DESCRIBE YOUR SOLUTION HERE !!!! DO NOT FORGET !!!! 👮🏻👮🏻👮🏻

Closes: #712
Solution:

• Integrate the option introduced in cosmos-sdk

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Thank you for your code, it's appreciated! :)

[JayT106]

LGTM