iolog: run compression work at slightly elevated nice level

Add support for work items to contain a priority, and use this
directly with nice. Run compression at nice 1, to make it slightly
less important than the actual IO jobs.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/lib/tp.h b/lib/tp.h
index 5b07cc6..9147cc2 100644
--- a/lib/tp.h
+++ b/lib/tp.h
@@ -10,6 +10,7 @@
 	struct flist_head list;
 	tp_work_fn *fn;
 	int wait;
+	int prio;
 	pthread_cond_t cv;
 	pthread_mutex_t lock;
 	volatile int done;