Merge pull request #977 from TheBlueMatt/2021-06-fix-double-claim-close

Handle double-HTLC-claims without failing the backwards channel

diff --cc lightning/src/chain/channelmonitor.rs
Simple merge

diff --cc lightning/src/ln/chanmon_update_fail_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

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

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