bench: fix loop starvation bug

Don't keep writing until the write queue fills up. On fast systems (mine), that
never happens - the data is sent out as fast as the benchmark generates it.

test/benchmark-pump.c

@@ -203,12 +203,10 @@ static void do_write(uv_stream_t* stream) {
   buf.base = (char*) &write_buffer;
   buf.len = sizeof write_buffer;
 
-  while (stream->write_queue_size == 0) {
   req = (uv_write_t*) req_alloc();
   r = uv_write(req, stream, &buf, 1, write_cb);
   ASSERT(r == 0);
 }
-}
 
 
 static void connect_cb(uv_connect_t* req, int status) {