Allow duplicate-payment_hash HTLCs for HTLC forwards

Allow duplicate-payment_hash HTLCs for HTLC forwards

This is required by BOLT 2 to ensure that no attacker can simply
relay every public node a duplicate-payment_hash HTLC for each HTLC
it receives to deduce where an HTLC came from.

Note that this makes the claim logic much less incentive-compatible
as we will not claim all available HTLCs with the same payment_hash
even if we know the preimage! This is OK because, most likely, any
attackers trying to map the network will use small-value payments
and, hopefully, we will move away from constant hashes across an
entire payment at some point in the near future.

This further simplifies the payment transition state a bit, so
hopefully at least we got some readability out of all of this