Bin batching: add some stats.

This lets us easily see what fraction of flush load is being taken up by the
bins, and helps guide future optimization approaches (for example: should we
prefetch during cache bin fills? It depends on how many objects the average fill
pops out of the batch).

include/jemalloc/internal/bin_stats.h

@@ -48,6 +48,11 @@ struct bin_stats_s {
 
 	/* Current size of nonfull slabs heap in this bin. */
 	size_t		nonfull_slabs;
+
+	uint64_t	batch_pops;
+	uint64_t	batch_failed_pushes;
+	uint64_t	batch_pushes;
+	uint64_t	batch_pushed_elems;
 };
 
 typedef struct bin_stats_data_s bin_stats_data_t;