Add incentive for verifying on-chain proof of solvency

[enricobottazzi]

This idea is inspired by the [Ethereum White Paper - Decentralized File Storage](https://ethereum.org/en/whitepaper/#decentralized-file-storage). In that implementation the storage of a file is split inside a Merkle tree and shared across multiple actors. To get a confirmation that the file is still being stored by someone, a smart contract picks a random number that represents an index inside the Merkle tree. The contract supplies x ether to the first actor able to provide the proof of ownership of the data block stored at that particular index of the Merkle tree. The goal here is to incentive actor to keep storing the data. The randomness of the request ensures that an actor must store the whole file to gain the reward.

One of the main problems of a Proof of Solvency protocol is that it requires users to actively verify their proof to attest to the exchange's solvency. The more users verify their proofs, the less likely it is that the exchange excludes users from their liabilities.

A similar mechanism can be applied to a Proof of Solvency protocol to incentivize users to verify their proof of solvency. Each user can be assigned an ID, and the smart contract can request a user with a specific ID (generated randomly starting from a block hash, for example) to provide for their proof of solvency in exchange for a reward.

The approach can add some privacy leaks to the protocol:

• The ID assigned to the users can indirectly reveal information about the number of exchange users
• The public address of the user that verifies the proof of chain gets publicly revealed. Everyone is now able to tell whether a public address is a customer of an exchange with a specific user ID