multi: extend lnpeer.Peer interface with Disconnect function

This will be used in the gossiper to disconnect from peers if their
ban score passes the ban threshold.

lnpeer/mock_peer.go

@@ -79,3 +79,5 @@ func (m *MockPeer) RemoteFeatures() *lnwire.FeatureVector {
 	args := m.Called()
 	return args.Get(0).(*lnwire.FeatureVector)
 }
+
+func (m *MockPeer) Disconnect(err error) {}

lnpeer/peer.go

@@ -74,4 +74,7 @@ type Peer interface {
 	// by the remote peer. This allows sub-systems that use this interface
 	// to gate their behavior off the set of negotiated feature bits.
 	RemoteFeatures() *lnwire.FeatureVector
+
+	// Disconnect halts communication with the peer.
+	Disconnect(error)
 }