Merge pull request #2816 from wpaulino/retryable-holder-sigs

Allow holder commitment and HTLC signature requests to fail

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/ln/functional_test_utils.rs
Simple merge