[Splicing] Make funding transaction available in funded channel

[optout21]

This is is a relatively minor change preparation for splicing ( #1621 ).

The funding transaction is available in the ChannelContext::funding_transaction field. However, it is available only during channel negtiation, it is cleared afterwards. In splicing, the funding transaction is needed later (for tx_add_input message).

Two possible approaches:

1. Safe (less invasive), but ugly (redundant): duplicate funding_transaction field, set it at the same time, but don't clear it. This approach is used in #3274 (proto).
2. Clean but risky: change the behavior that funding_transaction holds the transaction during the whole lifetime of the channel. The risk is that there might be code logic that relies on the field being cleared (which has to be found and adapted).