multi: update FetchChanInfos to take in an optional tx

In this commit, the FetchChanInfos ChannelGraph method is updated to
take in an optional read transaction for the case where it is called
from within another transaction.

discovery/chan_series.go

@@ -249,7 +249,7 @@ func (c *ChanSeries) FetchChanAnns(chain chainhash.Hash,
 		chanIDs = append(chanIDs, chanID.ToUint64())
 	}
 
-	channels, err := c.graph.FetchChanInfos(chanIDs)
+	channels, err := c.graph.FetchChanInfos(nil, chanIDs)
 	if err != nil {
 		return nil, err
 	}