Add 'sequential' file_service_type

This service type will keep a file open until IO to it is completely
done, before moving on to the next available file.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 4ff2d92..b8a7dd7 100644
--- a/HOWTO
+++ b/HOWTO
@@ -387,6 +387,10 @@
 			roundrobin  Round robin over open files. This
 				is the default.
 
+			sequential  Finish one file before moving on to
+				the next. Multiple files can still be
+				open depending on 'openfiles'.
+
 		The string can have a number appended, indicating how
 		often to switch to a new file. So if option random:4 is
 		given, fio will switch to a new random file after 4 ios