Add assertions for in-order block [dis]connection in ChannelManager

Sadly the connected-in-order tests have to be skipped in our normal
test suite as many tests violate it. Luckily we can still enforce
it in the tests which run in other crates.

Co-authored-by: Matt Corallo <git@bluematt.me>
Co-authored-by: Jeffrey Czyz <jkczyz@gmail.com>