Add logging for sampled allocations

- prof_opt_log flag starts logging automatically at runtime
- prof_log_{start,stop} mallctl for manual control

include/jemalloc/internal/extent_structs.h

@@ -161,11 +161,13 @@ struct extent_s {
 		/* Small region slab metadata. */
 		arena_slab_data_t	e_slab_data;
 
-		/*
-		 * Profile counters, used for large objects.  Points to a
-		 * prof_tctx_t.
-		 */
-		atomic_p_t		e_prof_tctx;
+		/* Profiling data, used for large objects. */
+		struct {
+			/* Time when this was allocated. */
+			nstime_t		e_alloc_time;
+			/* Points to a prof_tctx_t. */
+			atomic_p_t		e_prof_tctx;
+		};
 	};
 };
 typedef ql_head(extent_t) extent_list_t;