loopout: configurable payment timeout for off-chain payments

[bhandras]

With this client side RPC option the client will be able to set a payment timeout for individual off-chain payments. This applies both to the prepayment and the swap payment. With the swap payment, we may try to pay the swap invoice multiple times, but each attempt will use this configurable timeout.

Pull Request Checklist

• [ ] Update release_notes.md if your PR contains major features, breaking changes or bugfixes