Base the rwmix switch algorithm on io issues

The time based switch can be unfair sometimes, so just simplify it a bit
and use issues only.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 661dd49..036e62a 100644
--- a/fio.h
+++ b/fio.h
@@ -598,8 +598,7 @@
 	 * read/write mixed workload state
 	 */
 	os_random_state_t rwmix_state;
-	unsigned long long rwmix_bytes;
-	struct timeval rwmix_switch;
+	unsigned long rwmix_issues;
 	enum fio_ddir rwmix_ddir;
 	unsigned int ddir_nr;