Move background thread creation to background_thread_0.

To avoid complications, avoid invoking pthread_create "internally", instead rely
on thread0 to launch new threads, and also terminating threads when asked.

Author: interwq

include/jemalloc/internal/background_thread_externs.h

@@ -10,8 +10,6 @@ extern background_thread_info_t *background_thread_info;
 bool background_thread_create(tsd_t *tsd, unsigned arena_ind);
 bool background_threads_enable(tsd_t *tsd);
 bool background_threads_disable(tsd_t *tsd);
-bool background_threads_disable_single(tsd_t *tsd,
-    background_thread_info_t *info);
 void background_thread_interval_check(tsdn_t *tsdn, arena_t *arena,
     arena_decay_t *decay, size_t npages_new);
 void background_thread_prefork0(tsdn_t *tsdn);

include/jemalloc/internal/background_thread_structs.h

@@ -18,6 +18,8 @@ struct background_thread_info_s {
 	malloc_mutex_t		mtx;
 	/* Whether the thread has been created. */
 	bool			started;
+	/* Pause execution (for arena reset / destroy). */
+	bool			pause;
 	/* When true, it means no wakeup scheduled. */
 	atomic_b_t		indefinite_sleep;
 	/* Next scheduled wakeup time (absolute time in ns). */