Merge pull request #3153 from alecchendev/2024-07-async-closing-signed

Allow sending closing tx signatures asynchronously

diff --cc lightning/src/ln/async_signer_tests.rs
Simple merge

diff --cc lightning/src/ln/channel.rs
Simple merge

diff --cc lightning/src/ln/channelmanager.rs
Simple merge

diff --cc lightning/src/util/test_channel_signer.rs
Simple merge