The purpose of this interface is to distinguish messages that are
bound to a particular channel_id from those that are not. There is
no reason this ought to be a property of the peer package as it is
entirely determined by the contents of the message itself. Therefore
we move it to the lnwire package so it can be used without having
import cycles elsewhere in the codebase.
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.
