Correct final_cltv handling in blinded paths

When paying a blinded path, we don't have a CLTV delta at each hop
available, but rather only a total CLTV delta for the entire
blinded path.

However, the onion format currently still requires that we specify
an `outgoing_cltv_value` for the final hop. As the sender, we don't
have a sensible value to put there, as we don't know which part of
the total CLTV delta belongs to the recipient.

Thus, the sensible solution is to have the sender increment by the
amount they expect to receive, using zero otherwise. This works as
long as the sender isn't adding any additional CLTV delta for
privacy reasons. If the sender is doing so, they can use the final
CLTV field to communicate this.

This change proposes this, which appears to reflect what some
implementations do today.

04-onion-routing.md

@@ -262,6 +262,9 @@ The writer of the TLV `payload`:
       - MUST include the `blinding_point` provided by the recipient in `current_blinding_point`
     - If it is the final node:
       - MUST include `amt_to_forward`, `outgoing_cltv_value` and `total_amount_msat`.
+      - `outgoing_cltv_value` MUST be set to any additional CLTV delta which was added in addition
+        to the required amount for the full blinded path for privacy. This may be zero. This MUST
+        NOT include any additional amount added to compensate for a block during payment relay.
     - MUST NOT include any other tlv field.
   - For every node outside of a blinded route:
     - MUST include `amt_to_forward` and `outgoing_cltv_value`.
@@ -310,7 +313,7 @@ The reader:
       - MUST return an error if the payload contains other tlv fields than `encrypted_recipient_data`, `current_blinding_point`, `amt_to_forward`, `outgoing_cltv_value` and `total_amount_msat`.
       - MUST return an error if `amt_to_forward`, `outgoing_cltv_value` or `total_amount_msat` are not present.
       - MUST return an error if `amt_to_forward` is below what it expects for the payment.
-      - MUST return an error if incoming `cltv_expiry` < `outgoing_cltv_value`.
+      - MUST return an error if incoming `cltv_expiry` < `outgoing_cltv_value` + `min_final_cltv_expiry_delta`.
       - MUST return an error if incoming `cltv_expiry` < `current_block_height` + `min_final_cltv_expiry_delta`.
   - Otherwise (it is not part of a blinded route):
     - MUST return an error if `blinding_point` is set in the incoming `update_add_htlc` or `current_blinding_point` is present.