net/rpc: log Call reply discard

It means serious user error that can lead to
hard to debug issues under load, log entry
will not harm.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5574075

src/pkg/net/rpc/client.go

@@ -145,6 +145,7 @@ func (call *Call) done() {
 	default:
 		// We don't want to block here.  It is the caller's responsibility to make
 		// sure the channel has enough buffer space. See comment in Go().
+		log.Println("rpc: discarding Call reply due to insufficient Done chan capacity")
 	}
 }