multi: thread bandwidth check amount down to MayAddOutgoingHtlc

Pass htlc amount down to the channel so that we don't need to rely
on minHtlc (and pad it when the channel sets a 0 min htlc). Update
test to just check some sane values since we're no longer relying
on minHtlc amount at all.

peer/test_utils.go

@@ -445,7 +445,7 @@ func (m *mockUpdateHandler) Bandwidth() lnwire.MilliSatoshi { return 0 }
 func (m *mockUpdateHandler) EligibleToForward() bool { return false }
 
 // MayAddOutgoingHtlc currently returns nil.
-func (m *mockUpdateHandler) MayAddOutgoingHtlc() error { return nil }
+func (m *mockUpdateHandler) MayAddOutgoingHtlc(lnwire.MilliSatoshi) error { return nil }
 
 // ShutdownIfChannelClean currently returns nil.
 func (m *mockUpdateHandler) ShutdownIfChannelClean() error { return nil }