View function to check used proofs in ethereum without changing contract

[mfornet]

In order to check if a proof was already used we need to check in a dictionary inside an ethereum contract if the proof exists already.

Implement this as a function so it can be reused for several tools, in particular for frontend while restoring tx. Relevant comments: https://github.com/aurora-is-near/rainbow-bridge-frontend/issues/235#issuecomment-841173161

I'm thinking in the following interface:

function proofExists(web3, lockerAddress, proof) {
    // Return true if the proof already exists, false otherwise
}