[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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.88%. Comparing base (66fb520) to head (8339270). Report is 64 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3317      +/-   ##
==========================================
+ Coverage   89.66%   90.88%   +1.22%     
==========================================
  Files         126      127       +1     
  Lines      102676   115316   +12640     
  Branches   102676   115316   +12640     
==========================================
+ Hits        92062   104806   +12744     
+ Misses       7894     7869      -25     
+ Partials     2720     2641      -79     

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[optout21]

Note: Changing this behavior causes a single test case to fail: ln::payment_tests::test_completed_payment_not_retryable_on_reload (reproducible by not resetting funding_transaction in the original code, or keeping funding_transaction_rebroadcast_flag always false in the version of this PR)

The check that fails expects 1 broadcasted transaction, while with the change it will have the transaction broadcasted 4 times. I suspect this change does not really affect the desired behavior, this checks is simply over-specific and that can be updated, and the change is acceptable.

  assert_eq!(nodes[0].tx_broadcaster.txn_broadcasted.lock().unwrap().split_off(0).len(), 4);

[optout21]

Alternative simpler solution in #3380 -- never clears the funding_transaction field, no new fields, which may result in additional rebroadcasts. One should be chosen -- I lean for the simpler solution, but I have no strong opinion.

[TheBlueMatt]

Supersceded by #3380.