Set the default number of background threads to 4.

The setting has been tested in production for a while.  No negative effect while
we were able to reduce number of threads per process.

include/jemalloc/internal/background_thread_structs.h

@@ -9,6 +9,7 @@
 
 #define BACKGROUND_THREAD_INDEFINITE_SLEEP UINT64_MAX
 #define MAX_BACKGROUND_THREAD_LIMIT MALLOCX_ARENA_LIMIT
+#define DEFAULT_NUM_BACKGROUND_THREAD 4
 
 typedef enum {
 	background_thread_stopped,