background_thread: add max thread count config

Looking at the thread counts in our services, jemalloc's background thread
is useful, but mostly idle.  Add a config option to tune down the number of threads.

include/jemalloc/internal/background_thread_structs.h

@@ -8,6 +8,7 @@
 #endif
 
 #define BACKGROUND_THREAD_INDEFINITE_SLEEP UINT64_MAX
+#define MAX_BACKGROUND_THREAD_LIMIT MALLOCX_ARENA_LIMIT
 
 typedef enum {
 	background_thread_stopped,