Do additional pre-flight checks before claiming a payment

Do additional pre-flight checks before claiming a payment

As additional sanity checks, before claiming a payment, we check
that we have the full amount available in `claimable_htlcs` that
the payment should be for. Concretely, this prevents one
somewhat-absurd edge case where a user may receive an MPP payment,
wait many *blocks* before claiming it, allowing us to fail the
pending HTLCs and the sender to retry some subset of the payment
before we go to claim. More generally, this is just good
belt-and-suspenders against any edge cases we may have missed.