event: store the outpoint when is_manual_broadcast

event: store the outpoint when is_manual_broadcast

With [1], it's possible to specify `manual_broadcast` for
the channel funding transaction. When `is_manual_broadcast` is
set to true, the transaction in the `DiscardFunding` event is
replaced with a dummy empty transaction.

This commit checks if `is_manual_broadcast` is true and
stores the funding OutPoint in the DiscardFunding event instead.

[1] https://github.com/lightningdevkit/rust-lightning/pull/3024

Link: https://github.com/lightningdevkit/rust-lightning/issues/3164
Suggested-by: TheBlueMatt
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>