Close channel with one transaction

[JosephGoulden]

At the moment closing a channel produces a spendable output which is then sent to the BDK wallet. This results in two transactions when one should be sufficient. So either we need to supply the channel close output to BDK or better yet set the output destination when we open the channel (for increased security).

Return closing transaction ID in API response.

Use KeysInterface::get_shutdown_scriptpubkey to specify closing address when channel is opened.