Add option for including byte offset for each log entry

When iops/latency/bw logs are used in the job, we can note the offset
at which the specific data entry took place.

Based on an initial patch from Abutalib Aghayev <agayev@gmail.com>.
Extended by me to:

- Not do this by default, as not to add 8 bytes to each log entry
  stored.

- Add support for the client/server protocol.

- Add documentation.

We should bump the server protocol rev again, but that was just done
in the previous commit, so...

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/HOWTO b/HOWTO
index d9f3095..3001fe7 100644
--- a/HOWTO
+++ b/HOWTO
@@ -1333,6 +1333,9 @@
 		specified period of time, reducing the resolution of the log.
 		Defaults to 0.
 
+log_offset=int	If this is set, the iolog options will include the byte
+		offset for the IO entry as well as the other data values.
+
 lockmem=int	Pin down the specified amount of memory with mlock(2). Can
 		potentially be used instead of removing memory or booting
 		with less memory to simulate a smaller amount of memory.