Add max_per_bg_thd stats for per background thread mutexes.

Added a new stats row to aggregate the maximum value of mutex counters for each
background threads.  Given that the per bg thd mutex is not expected to be
contended, this counter is mainly for sanity check / debugging.

include/jemalloc/internal/background_thread_structs.h

@@ -48,6 +48,7 @@ struct background_thread_stats_s {
 	size_t num_threads;
 	uint64_t num_runs;
 	nstime_t run_interval;
+	mutex_prof_data_t max_counter_per_bg_thd;
 };
 typedef struct background_thread_stats_s background_thread_stats_t;