Add version 2 of the iolog format

The old format had some short comings - notably not supporting > 1
file workloads. So add a new format that defines adding/open/close
of files. Fio will still load the older format iologs, but the new
ones wont work on older fio versions.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index b636f57..309969a 100644
--- a/io_u.c
+++ b/io_u.c
@@ -357,9 +357,7 @@
 	 * If using a write iolog, store this entry.
 	 */
 out:
-	if (td->o.write_iolog_file)
-		write_iolog_put(td, io_u);
-
+	log_io_u(td, io_u);
 	return 0;
 }