Low-Risk/QA01: Add isAllowedToken() modifier to liquidate()

affected line: L231

Low-Risk/QA02: Consider making stale TIMEOUT check much shorter than 3 hours (ex: 15 minutes)

Low-Risk/QA03: Having a single EOA as the only owner of contracts is a large centralization risk and a single point of

failure. A single private key may be taken in a hack, or the sole holder of the key may become unable to

retrieve the key when necessary. Consider changing to a multi-signature setup, or having a role-based

authorization model.