Merge pull request #3063 from jirijakes/upgrade-bitcoin-031

Upgrade rust-bitcoin dependency to 0.31

diff --cc lightning/src/events/mod.rs
Simple merge

diff --cc lightning/src/ln/chanmon_update_fail_tests.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/monitor_tests.rs
Simple merge

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

diff --cc lightning/src/routing/router.rs
Simple merge