In this commit, we export the `ResolveContract` method as it's useful as
a way to manually remove active contracts from the chain and channel
arbitrator. Along the way, we also update the method to also attempt to
stop the channel arb if it exists. This allows an external party to
remove all state with a single call. Before this commit, it was assumed
that this method was only called by the channel arb itself, when it was
already on the way to exiting after all contracts were fully resolved.
We also add a set of unit tests to exercise the intended behavior as
this method is now public.
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