Change default chunk size from 4 MiB to 256 KiB.

Recent changes have improved huge allocation scalability, which removes
upward pressure to set the chunk size so large that huge allocations are
rare.  Smaller chunks are more likely to completely drain, so set the
default to the smallest size that doesn't leave excessive unusable
trailing space in chunk headers.

include/jemalloc/internal/chunk.h

@@ -5,7 +5,7 @@
  * Size and alignment of memory chunks that are allocated by the OS's virtual
  * memory system.
  */
-#define	LG_CHUNK_DEFAULT	22
+#define	LG_CHUNK_DEFAULT	18
 
 /* Return the chunk address for allocation address a. */
 #define	CHUNK_ADDR2BASE(a)						\