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/thread_options.h b/thread_options.h
index 57d84db..e53000a 100644
--- a/thread_options.h
+++ b/thread_options.h
@@ -108,6 +108,7 @@
 	unsigned long long rand_seed;
 	unsigned int use_os_rand;
 	unsigned int log_avg_msec;
+	unsigned int log_offset;
 	unsigned int norandommap;
 	unsigned int softrandommap;
 	unsigned int bs_unaligned;
@@ -335,6 +336,7 @@
 	uint64_t rand_seed;
 	uint32_t use_os_rand;
 	uint32_t log_avg_msec;
+	uint32_t log_offset;
 	uint32_t norandommap;
 	uint32_t softrandommap;
 	uint32_t bs_unaligned;