i read this protocol and i still have some questions for you 1) what is the difference between a liquidity provider and a traditional exchange? seems they both do the same thing 2) how does this payment channel work? i read the hash time locked contract which simply means if alice sends bob some money, bob needs his private key to get the money within the time set by alice or else alice gets back the money, but how is bob s key being used across multiple transactions on the COMIT network?