peers: add a funding_tx_id field to funding_locked2

As the `channel_id` no longer includes the tx hash of the confirmed
funding transaction, confirm the funding transaction hash in the
`funding_locked2` message. In the rare event of a network partition
or block reorg, this might be handy.

Author: niftynei

02-peer-protocol.md

@@ -693,11 +693,24 @@ transaction and maintains information symmetry.
 1. type: 61 (`funding_locked2`)
 2. data:
   * [`32`:`channel_id`]
+  * [`32`:`funding_tx_id`]
   * [`33`:`next_per_commitment_point`]
 
-#### Requirements and Rationale
+See [`funding_locked`](#the-funding_locked-message) for Requirements and
+Rationale, with the following addendum.
 
-See [`funding_locked`](#the-funding_locked-message).
+#### Requirements
+
+The receiving node:
+  - if the `funding_tx_id` does not match your `funding_tx_id`:
+    - MUST fail the channel.
+
+#### Rationale
+
+`funding_tx_id` is the transaction hash for the funding transaction that
+is being locked in. This is to confirm that both nodes are using the same
+transaction for funding. This guards against a conflict in the case of a
+network partition.
 
 
 ### Kicking Off Replace-By-Fee: `init_rbf`