In this commit, we update the `AbandonChannel` method to also remove the
state from the countract court as well as the channel graph. Abandoning
a channel is now a three step process: remove from the open channel
state, remove from the graph, remove from the contract court. Between
any step it's possible that the users restarts the process all over
again. As a result, each of the steps below are intended to be
idempotent.
We also update the integration test to assert that no channel is found
in the graph any longer. Before this commit, this test would fail as the
channel was still found in the graph, which can cause other issues for
an operational daemon.
Fixes#3716.
32965fd4be