lnrpc: making PendingChannels req/resp naming scheme consistent

In this commit we rename the lnrpc.PendingChannelRequest and
lnrpc.PendingChannelResponse to
lnrpc.PendingChannelsRequest/lnrpc.PendingChannelsResponse. We do this
as we strive to ensure that the naming scheme across the RPC interface
is consistent.

cmd/lncli/commands.go

@@ -901,7 +901,7 @@ func pendingChannels(ctx *cli.Context) error {
 	client, cleanUp := getClient(ctx)
 	defer cleanUp()
 
-	req := &lnrpc.PendingChannelRequest{}
+	req := &lnrpc.PendingChannelsRequest{}
 	resp, err := client.PendingChannels(ctxb, req)
 	if err != nil {
 		return err