[Splicing] Preserve funding_transaction for the later lifecycle of the channel

[optout21]

Fixes #3300

In splicing, the funding transaction (not just the ID) is needed during splice negotiation.
Funding transaction is kept in field ChannelContext.funding_transaction.
However, the way it is currently used is that it is cleared when the tx is broadcast, and this fact is used in some logic.

This change:

• does not clear ChannelContext.funding_transaction when tx is broadcast, but it's preserved
• adds a funding_transaction_broadcast bool field to indicate if it has been broadcast

So behavior of not (re)broadcasting twice is preserve (though I am not sure whether this was intentional and it is needed, one test fails if this is changed (test_completed_payment_not_retryable_on_reload)).

Alternative would be to leave funding_transaction unchanged, and introduce a new funding_transaction_saved field, that is set the same way, but never cleared. This way existing logic would not be affected, but the tx would be kept in two copies.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.63%. Comparing base (66fb520) to head (bb043a1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3317      +/-   ##
==========================================
- Coverage   89.64%   89.63%   -0.02%     
==========================================
  Files         126      126              
  Lines      102676   102697      +21     
  Branches   102676   102697      +21     
==========================================
+ Hits        92045    92048       +3     
- Misses       7912     7920       +8     
- Partials     2719     2729      +10     

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