htlcswitch: add new method to the ChannelLink interface, EligibleToForward

In this commit, we add a new method to the ChanneLink interface:
EligibleToForward. This method allows a link to be added to the switch,
but in an intermediate state which indicates that it isn’t yet ready to
forward any incoming HTLC’s.

htlcswitch/mock.go

@@ -430,6 +430,7 @@ func (f *mockChannelLink) Bandwidth() lnwire.MilliSatoshi     { return 99999999
 func (f *mockChannelLink) Peer() Peer                         { return f.peer }
 func (f *mockChannelLink) Start() error                       { return nil }
 func (f *mockChannelLink) Stop()                              {}
+func (f *mockChannelLink) EligibleToForward() bool            { return true }
 
 var _ ChannelLink = (*mockChannelLink)(nil)