Improve shield purchases and claims

[dshe-certik]

Summary

When we create or update a pool, a purchase is made.

• [ ] The purchaser is set to be shield admin rather than sponsor address. Considering the sponsor is the one who submits claims, it should be the sponsor address.
• [ ] No purchase event is emitted. Should we move the purchase event from the handler to the keeper?
• [ ] We should not allow adding service fees without any purchases, because service fees are updated when purchases expire.

When a proposal is submitted,

• [ ] we didn't consider the case when we can't find the target purchase: https://github.com/certikfoundation/shentu/blob/master/x/shield/keeper/proposal.go#L52

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For Admin Use

• [ ] Not duplicate issue
• [ ] Appropriate labels applied
• [ ] Appropriate contributors tagged

[jim380]

If a purchase couldn't be found with the provided poolID and purchaser, it would've errored out on L42, no?

[dshe-certik]

If a purchase couldn't be found with the provided poolID and purchaser, it would've errored out on L42, no?

Right, but we didn't check if the target purchase ID exists or not for this purchaser.