Merge pull request #851 from TheBlueMatt/2021-03-holding-cell-clear-msg-get

Clean up and more liberally free holding cell HTLCs (without re-entrancy)

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