Store an `events::PaymentPurpose` with each claimable payment

Store an `events::PaymentPurpose` with each claimable payment

In fc77c57c3c6e165d26cb5c1f5d1afee0ecd02589 we stopped using the
`FInalOnionHopData` in `OnionPayload::Invoice` directly and intend
to remove it eventually. However, in the next few commits we need
access to the payment secret when claimaing a payment, as we create
a new `PaymentPurpose` during the claim process for a new event.

In order to get access to a `PaymentPurpose` without having access
to the `FinalOnionHopData` we here change the storage of
`claimable_htlcs` to store a single `PaymentPurpose` explicitly
with each set of claimable HTLCs.