Update PayByContract to return 'false' if a contract has reached the max revision number

[ChrisSchinnerl]

Not sure if this is the right place to put this considering that the return type is a boolean, but some kind of safeguard against this seems appropriate.

[lukechampine]

Having false represent multiple distinct types of failure is suspicious. Case in point, it looks like renterd currently does:

        payment, ok := rhpv3.PayByContract(rev, amount, refundAcct, sk)
        if !ok {
                return rhpv3.PayByContractRequest{}, ErrInsufficientFunds
        }

which would be incorrect behavior if !ok can mean "contract not revisable." So this should probably return an error instead.

The other option would be to match the API of PayByEphemeralAccount: instead of failing, return a revision that the host will (presumably) reject. But I think it's better to catch errors locally when possible.

[lukechampine]

@ChrisSchinnerl are you still interested in merging this, or is renterd using a different approach now?