watchtower/wtclient: integrate ClientChannelSummaries

In this commit, we utilize the more generic ClientChanSummary instead of
exposing methods that only allow us to set and fetch sweep pkscripts.

watchtower/wtclient/client_test.go

@@ -605,6 +605,8 @@ func (h *testHarness) backupStates(id, from, to uint64, expErr error) {
 // backupStates instructs the channel identified by id to send a backup for
 // state i.
 func (h *testHarness) backupState(id, i uint64, expErr error) {
+	h.t.Helper()
+
 	_, retribution := h.channel(id).getState(i)
 
 	chanID := chanIDFromInt(id)