Buyer and seller can replace moderator

[sameepsi]

Our current version of escrow smart contract does not allow to replace moderator once the transaction is created.
Whereas Buyer and Seller can mutually agree on changing the moderator if they find the current moderator to be malicious or because of any other reasons.

Ideally, we should collect any authorizing signatures off-chain, via OpenBazaar, so that either the buyer or seller can make 1 function call to the smart contract (with signature data and moderator account as arguments) to induce the change.

We are using similar off chain signing mechanism for executing transactions currently.

This enhancement was initially suggested by @drwasho