watchtower: start using the new channel max heights

This commit also adds tests for the DB changes made in the previous
commit since we can now read the new field with the FetchChanInfos
method.

The commit following this one does the backfill migration.

watchtower/wtclient/interface.go

@@ -81,10 +81,10 @@ type DB interface {
 	// successfully backed up using the given session.
 	NumAckedUpdates(id *wtdb.SessionID) (uint64, error)
 
-	// FetchChanSummaries loads a mapping from all registered channels to
-	// their channel summaries. Only the channels that have not yet been
+	// FetchChanInfos loads a mapping from all registered channels to
+	// their wtdb.ChannelInfo. Only the channels that have not yet been
 	// marked as closed will be loaded.
-	FetchChanSummaries() (wtdb.ChannelSummaries, error)
+	FetchChanInfos() (wtdb.ChannelInfos, error)
 
 	// MarkChannelClosed will mark a registered channel as closed by setting
 	// its closed-height as the given block height. It returns a list of