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/wtdb/client_chan_summary.go

@@ -1,11 +1,18 @@
 package wtdb
 
 import (
+	"errors"
 	"io"
 
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
+var (
+	// ErrChannelAlreadyRegistered signals a duplicate attempt to
+	// register a channel with the client database.
+	ErrChannelAlreadyRegistered = errors.New("channel already registered")
+)
+
 // ChannelSummaries is a map for a given channel id to it's ClientChanSummary.
 type ChannelSummaries map[lnwire.ChannelID]ClientChanSummary