rio fdset target: handle short writes.

While the socket is set in blocking mode, we still can get short writes
writing to a socket.

src/replication.c

@@ -888,6 +888,7 @@ void readSyncBulkPayload(aeEventLoop *el, int fd, void *privdata, int mask) {
     }
 
     nread = read(fd,buf,readlen);
+    printf("NREAD %d (%d)\n", (int)nread, (int)readlen);
     if (nread <= 0) {
         redisLog(REDIS_WARNING,"I/O error trying to sync with MASTER: %s",
             (nread == -1) ? strerror(errno) : "connection lost");