Implement updated retransmission logic for splice_locked

[t-bast]

While working on taproot tests for splicing, @sstone discovered that what was previously a harmless race condition actually becomes an issue when using taproot channels. This is detailed in lightning/bolts#1223 and lead to changes in the reestablish logic to fix that race condition, which was added to the splicing spec in lightning/bolts@2c1b500

I think we should implement those changes immediately in eclair and lightning-kmp, with the following tweaks for backwards-compatibility:

• we should handle reading the my_current_funding_locked
• if it is included, we should apply the requirements laid out in lightning/bolts@2c1b500
• if it isn't included, we should keep retransmitting our previous splice_locked like we currently do
• but we shouldn't set my_current_funding_locked yet, because current nodes don't know how to read it and it has an even type
  • we will start setting it in Use final spec values for splicing #2887 when the remote node supports the official splicing feature bit