Add a `PaymentId` for inbound payments

Add a `PaymentId` for inbound payments

We expect our users to have fully idempotent `Event` handling as we
may replay events on restart for one of a number of reasons. This
isn't a big deal as long as all our events have some kind of
identifier users can use to check if the `Event` has already been
handled.

For outbound payments, this is the `PaymentId` they provide in the
send methods, however for inbound payments we don't have a great
option.

`PaymentHash` largely suffices - users can simply always claim in
response to a `PaymentClaimable` of sufficient value and treat a
`PaymentClaimed` event as duplicate any time they see a second one
for the same `PaymentHash`. This mostly works, but may result in
accepting duplicative payments if someone (incorrectly) pays twice
for the same `PaymentHash`.

Users could also fail for duplicative `PaymentClaimable` events of
the same `PaymentHash`, but doing so may result in spuriously
failing a payment if the `PaymentClaimable` event is a replay and
they never saw a corresponding `PaymentClaimed` event.

While none of this will result in spuriously thinking they've been
paid when they have not, it does result in some pretty awkward
semantics which we'd rather avoid our users having to deal with.

Instead, here, we add a new `PaymentId` which is simply an HMAC of
the HTLCs (as Channel ID, HTLC ID pairs) which were included in the
payment.