bolt2: allow multiple shutdown message from the sender side.

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

02-peer-protocol.md

@@ -544,6 +544,7 @@ A sending node:
   - MAY send a `shutdown` before a `funding_locked`, i.e. before the funding transaction has reached `minimum_depth`.
   - if there are updates pending on the receiving node's commitment transaction:
     - MUST NOT send a `shutdown`.
+  - MAY send multiple `shutdown` message, (but what happens if in the two shutdown message we insert different script? We need to accept only the last one sent?).
   - MUST NOT send an `update_add_htlc` after a `shutdown`.
   - if no HTLCs remain in either commitment transaction:
     - MUST NOT send any `update` message after a `shutdown`.
@@ -570,6 +571,7 @@ A receiving node:
   - if both nodes advertised the `option_upfront_shutdown_script` feature, and the receiving node received a non-zero-length `shutdown_scriptpubkey` in `open_channel` or `accept_channel`, and that `shutdown_scriptpubkey` is not equal to `scriptpubkey`:
     - MAY send a `warning`.
     - MUST fail the connection.
+  - MUST allow multiple `shutdown` messages from the sender.
 
 #### Rationale