Set huge_threshold to 8M by default.

This feature uses an dedicated arena to handle huge requests, which
significantly improves VM fragmentation.  In production workload we tested it
often reduces VM size by >30%.

include/jemalloc/internal/arena_types.h

@@ -46,6 +46,6 @@ typedef enum {
  * When allocation_size >= huge_threshold, use the dedicated huge arena (unless
  * have explicitly spicified arena index).  0 disables the feature.
  */
-#define HUGE_THRESHOLD_DEFAULT 0
+#define HUGE_THRESHOLD_DEFAULT (8 << 20)
 
 #endif /* JEMALLOC_INTERNAL_ARENA_TYPES_H */