Adds a new Brontide struct method tryLinkShutdown that attempts to
fetch the target link and calls ShutdownIfChannelClean on it. This
allows the coop close process to guarantee atomicity of the underlying
channel state. Also removes the UnregisterChannel method from the
chancloser's config as the link is shut down before the chancloser
is created.
This commit allows the peer to be tested without relying on a raw
htlcswitch.Switch pointer. This is accomplished by using a messageSwitch
interface and adding the CreateAndAddLink method to the
htlcswitch.Switch.
This commit moves all localized instances of mock implementations of
the Signer interface to the lntest/mock package. This allows us to
remove a lot of code and have it housed under a single interface in
many cases.
