Give users who use `get_payment_secret_preimage` the PaymentPreimage

Give users who use `get_payment_secret_preimage` the PaymentPreimage

For users who get PaymentPreimages via
`get_payment_secret_preimage`, they need to provide the
PaymentPreimage back in `claim_funds` but they aren't actually
given the preimage anywhere.

This commit gives users the PaymentPreimage in the
`PaymentReceived` event.