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/options.c b/options.c
index 74347f3..6d326d4 100644
--- a/options.c
+++ b/options.c
@@ -3095,6 +3095,16 @@
 		.group	= FIO_OPT_G_INVALID,
 	},
 	{
+		.name	= "log_offset",
+		.lname	= "Log offset of IO",
+		.type	= FIO_OPT_BOOL,
+		.off1	= td_var_offset(log_offset),
+		.help	= "Include offset of IO for each log entry",
+		.def	= "0",
+		.category = FIO_OPT_C_LOG,
+		.group	= FIO_OPT_G_INVALID,
+	},
+	{
 		.name	= "bwavgtime",
 		.lname	= "Bandwidth average time",
 		.type	= FIO_OPT_INT,