Update airdrop contract

[hyeonLewis]

Proposed changes

Update Airdrop.sol contract.

1. Add claimAllowed to control claim.
2. Add receiver function to allow airdrop to receive funds.

• This is because we may need to send funds to Airdrop not through state modification.

3. Owner can override registered claim.

• Note that it doesn't change claimed, but only claims.

5. Tidy comments and functions.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

Checklist

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• I have read the CLA and signed by comment I have read the CLA Document and I hereby sign the CLA in first time contribute
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

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Further comments

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